Astronomy masterclass 2015

Evolution of a star

The next session of the day was looking at stars and their evolution


The session was run by Megan Whewell, who is studying for a PhD, with a focus on black holes at the centre of galaxies, at the Mullard Space Science Laboratory at the University College London (MSSL)

Megan started off by explaining the Hertzsprung-Russell diagram


The Hertzsprung–Russell diagram, abbreviated H–R diagram or HRD, is a scatter graph of stars showing the relationship between the stars’ absolute magnitudes or luminosities versus their spectral classifications or effective temperatures. More simply, it plots each star on a graph measuring the star’s brightness against its temperature (colour). It does not map any locations of stars.

The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution or “the way in which stars undergo sequences of dynamic and radical changes over time”.


Ejnar Hertzsprung, shown above left, (8 October 1873 – 21 October 1967) was a Danish chemist and astronomer.

Henry Norris Russell, shown above right, (October 25, 1877 – February 18, 1957) was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung–Russell diagram (1910).


The Hertzsprung-Russell diagram (HR diagram) is one of the most important tools in the study of stellar evolution. Developed independently in the early 1900s by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity (the theoretical HR diagram), or the colour of stars (or spectral type) against their absolute magnitude (the observational HR diagram, also known as a colour-magnitude diagram).

Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves. This reveals the true power of the HR diagram – astronomers can know a star’s internal structure and evolutionary stage simply by determining its position in the diagram.


The letters indicate the spectral classes of the stars under the Morgan–Keenan (MK) system – a sequence from the hottest (O type) to the coolest (M type).

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Light from the star is analysed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colours interspersed with absorption lines. Each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere. The spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere’s temperature and density.

The Hertzsprung-Russell diagram shows the various stages of stellar evolution. By far the most prominent feature is the main sequence (grey), which runs from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) of the diagram. The giant branch and supergiant stars lie above the main sequence, and white dwarfs are found below it. Credit: R. Hollow, CSIRO.

The Hertzsprung-Russell diagram above shows a group of stars in various stages of their evolution. By far the most prominent feature is the main sequence, which runs from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) of the diagram. The giant branch is also well populated and there are many white dwarfs. Also plotted are the Morgan-Keenan luminosity classes that distinguish between stars of the same temperature but different luminosity.

There are 3 main regions (or evolutionary stages) of the HR diagram:

The main sequence stretching from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) dominates the HR diagram. It is here that stars spend about 90% of their lives burning hydrogen into helium in their cores. Main sequence stars have a Morgan-Keenan luminosity class labelled V.

Red giant and supergiant stars (luminosity classes I through III) occupy the region above the main sequence. They have low surface temperatures and high luminosities which, according to the Stefan-Boltzmann law, means they also have large radii. Stars enter this evolutionary stage once they have exhausted the hydrogen fuel in their cores and have started to burn helium and other heavier elements.

Stars are considered to be black bodies, i.e. they are perfect emitters of electromagnetic radiation and the Stefan-Boltzmann says that the luminosity of a star depends on its surface area and the fourth power of its absolute temperature.

L = σAT^4 the Greek letter sigma (σ) represents the constant of proportionality, called the Stefan–Boltzmann constant. A is the surface area of a sphere (4pr^2) where r is the radius of the star.

White dwarf stars (luminosity class D) are the final evolutionary stage of low to intermediate mass stars, and are found in the bottom left of the HR diagram. These stars are very hot but have low luminosities due to their small size.

The Sun is found on the main sequence with a luminosity of 1 and a temperature of around 5,400 Kelvin.

Astronomers generally use the HR diagram to either summarise the evolution of stars, or to investigate the properties of a collection of stars. In particular, by plotting a HR diagram for either a globular or open cluster of stars, astronomers can estimate the age of the cluster from where stars appear to turn off the main sequence.

Stars form in regions of space where there are huge quantities of gas and dust called nebulae. Each nebula is composed of about 75% hydrogen and 23% helium. The other 2% is oxygen, nitrogen, carbon, and silicate dust. Some of this interstellar matter came from exploding stars.


The Eagle Nebula in visible light, along with the iconic Hubble Space Telescope image of the centre (top-right). The near infrared light, from the VLT (bottom right), shows only the densest parts, within which stars are forming. Image credit: MPG/ESO (main image); NASA/ESA/STScI (Hubble); VLT/ISAAC/AIP/ESO (near-infrared)


Views of the Eagle Nebula in visible light can only tell astronomers so much. Cold dust, made up of small grains of carbon and silicates blocks out the light from stars within or beyond the pillars, with the only illumination coming from the outer layers of gas that are energised by the intense light from nearby stars. The Herschel Space Observatory, meanwhile, sees far-infrared light, with wavelengths thousands of times longer than visible light. Rather than seeing the pillars as dark silhouettes, Herschel sees the clouds of dust glowing in their own light.

Our Sun is about 4.6 billion years old and is considered middle aged. It began as a region of gas several times larger than the current solar system.

This region of gas collapsed due to a gravitational attraction acting between the atoms of gas and grains of dust and a rotating cloud of gas and dust formed. As more material was drawn into the spinning ball, the mass at its core increased and the temperature climbed. When the core got hot enough, it started to glow and a protostar formed. This was the first stage in our Sun’s formation

After about 2000 years the surface temperature reached 3000K simply due to the gravitational contraction.

Birth of a planetary system: (a) Self-gravity collapses a slowly rotating cloud of interstellar gas and dust. (b) The cloud flattens and rotates faster around a newly formed protostar. (3) The young star begins to shine surrounded by a flattened disk out of which planets will eventually form.


As the process of Sun building continued, the interior of the protostar got hotter and hotter and when the temperature reached 10000000 degrees C hydrogen started to change to helium. This process, known as fusion, released great quantities of energy and radiation. Our Sun was born.

Any remaining gas and dust formed the planets, moons, asteroids, comets and meteors (meteorites) orbiting the Sun.


The image below left shows Megan holding a tiny piece of the Solar System – a meteorite (I hope she will forgive the terrible picture but the room was dark).


A meteorite is a solid piece of debris, from such sources as asteroids or comets that originates in outer space and survives its impact with the Earth’s surface.

Hubble imaging nebulae


Orion Nebula M42, NGC 1976; Credit: NASA, ESA, T. Megeath (University of Toledo) and M. Robberto (STScI)


The above image shows that the Orion nebula is just below Orion’s belt

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way south of Orion’s Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of 1,344 ± 20 light years and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula

The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula.


A reflection nebula in Orion – Credit:

NASA/ESA and the Hubble Heritage Team (STScI)

In astronomy, reflection nebulae are clouds of interstellar dust which reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized.


The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990, and remains in operation. With a 2.4-meter mirror, Hubble’s four main instruments observe in the near ultraviolet, visible, and near infrared spectra. The telescope is named after the astronomer Edwin Hubble.

Herschel imaging nebulae




The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched, carrying a single 3.5-metre mirror and instruments sensitive to the far infrared and submillimetre wavebands (55–672 µm). Herschel was the fourth cornerstone mission in the ESA science programme, along with Rosetta, Planck, and Gaia. NASA is a partner in the Herschel mission, with US participants contributing to the mission; providing mission-enabling instrument technology and sponsoring the NASA Herschel Science Center (NHSC) at the Infrared Processing and Analysis Center and the Herschel Data Search at the Infrared Science Archive.

The observatory was capable of seeing the coldest and dustiest objects in space; for example, cool cocoons where stars form and dusty galaxies just starting to bulk up with new stars. The observatory sifted through star-forming clouds—the “slow cookers” of star ingredients—to trace the path by which potentially life-forming molecules, such as water, form.

Herschel was charged with four primary areas of investigation:

Galaxy formation in the early universe and the evolution of galaxies;

Star formation and its interaction with the interstellar medium;

Chemical composition of atmospheres and surfaces of Solar System bodies, including planets, comets and moons;

Molecular chemistry across the universe

Herschel was instrumental in the discovery of an unknown and unexpected step in the star forming process. The initial confirmation and later verification via help from ground based telescopes of a vast hole of empty space, previously believed to be a dark nebula, in the area of NGC 1999 shed new light in the way newly forming star regions discard the material which surround them.

Back to the Hertsprung-Russell diagram


In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar colour versus brightness.. Stars on this band are known as main-sequence stars or “dwarf” stars.

After a star has formed, it generates thermal energy in the dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star’s lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. All main-sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of energy generation in the core on the temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both.

The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun (or 1.5 solar masses (M☉)) primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases, while main-sequence stars below 0.4 M☉ undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.

A star remains near its initial position on the main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen-burning stage of a star’s lifetime.

In general, the more massive a star is, the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram. The behaviour of a star now depends on its mass, with stars below 0.23 M☉ becoming white dwarfs directly, while stars with up to ten solar masses pass through a red giant stage. More massive stars can explode as a supernova, or collapse directly into a black hole.

Nuclear fusion in stars with hydrogen

Four hydrogen nuclei are needed along with high temperatures (to provide enough energy for electrostatic repulsion to be overcome) and high density/pressures to ensure that collisions between the hydrogen nuclei take place.

The process of hydrogen fusion – the proton proton chain

Two protons (hydrogen nuclei) fuse to form heavy hydrogen (deuterium)


A heavy hydrogen fuses with a proton to make helium 3

Two helium 3’s fuse to form helium 4 and two protons

The mass of the products of this reaction are slightly less than the mass of what we started with. The energy from the reaction comes from the conversion of mass into energy.

The mass of a Hydrogen nucleus is 1.6725 X E-27 kg and the mass of a Helium nucleus is 6.644 X E-27 kg. Four H nuclei have a combined mass of 6.69 X E-27 kg. A He nucleus has a mass of 6.644 X E-27 kg, so there is a mass deficit of 0.046 X E-27 kg – this tiny bit of mass has been turned into energy. Using


you can conclude that the amount of energy released is:


Now this quantity of energy might not seem much but there are about 9.29 x E37 of these reactions per second and the total luminosity (energy radiated per second) of the Sun is about 3.85 x E26 J/s. The Sun loses about 4.27 x E9 kg every second and will spend about 10 billion years on the main sequence.

While on the Main Sequence stars, including our Sun, are busily turning Hydrogen into Helium. This occurs in the core of the star and proceeds until about 10% of the star’s Hydrogen is converted to Helium. This defines the main sequence lifetime phase for a star.


Believed to be the most massive and brightest star – blue hypergiant

R136a1 (RMC 136a1) is a Wolf-Rayet star located near the centre of R136, the central condensation of stars of the large NGC 2070 open cluster in the Tarantula Nebula, a giant H II region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. It has the highest confirmed mass and bolometric luminosity of any known star, at 256 M☉ and 7.4 million L☉, as well as one of the highest surface temperatures of any main sequence star, at more than 55,000 K. It lies at a distance of about 48 kiloparsecs (157,000 light-years) based on the currently accepted distance to R136. Because of its large mass, it is one of the candidates for a potential supernova or hypernova in the astronomically near future.


Believed to be the biggest star

VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. It is one of the largest stars (at one time the largest known) and also one of the most luminous of its type, and has a radius of approximately 1,420 ± 120 solar radii[8] (equal to a diameter of 13.2 astronomical units, or about 1,976,640,000 km), and is located about 1.2 kiloparsecs (3,900 light-years) from Earth. VY Canis Majoris is a single star categorized as a semiregular variable with an estimated period of 2,000 days. It has an average density of 5 to 10 mg/m3. If placed at the centre of the Solar System, VY Canis Majoris’s surface would extend beyond the orbit of Jupiter, although there is still considerable variation in estimates of the radius, with some making it larger than the orbit of Saturn



Bellatrix, also known by its Bayer designation Gamma Orionis, is the third brightest star in the constellation Orion, 5° right of the red giant α Ori (Betelgeuse). Just between the 1st and 2nd magnitude, it is the 27th brightest star in the night sky.


From left to right, the stars Bellatrix, the Sun, and Algol B

Bellatrix is a massive star with about 8.4 times the Sun’s mass. It has an estimated age of approximately 20 million years; long enough for a star of this mass to consume the hydrogen at its core and begin to evolve away from the main sequence into a giant star. The effective temperature of the outer envelope of this star is 22000 K, which is considerably hotter than the 5,778 K on the Sun. This high temperature gives this star the blue-white hue that occurs with B-type stars. The measured angular diameter of this star, after correction for limb darkening, is 0.72 ± 0.04 mas. At an estimated distance of 250 light-years (77 parsecs), this yields a physical size of about six times the radius of the Sun.

Rigel, also known by its Bayer designation Beta Orionis (β Ori, β Orionis), is the brightest star in the constellation Orion and the seventh brightest star in the night sky, with visual magnitude 0.13. The star as seen from Earth is actually a triple star system, with the primary star (Rigel A) a blue-white supergiant of absolute magnitude −7.84 and around 120000 times as luminous as the Sun. An Alpha Cygni variable, it pulsates periodically. Visible in small telescopes, Rigel B is itself a spectroscopic binary system, consisting of two main sequence blue-white stars of spectral type B9.


Saiph is the sixth-brightest star in the constellation of Orion. Of the four bright stars that compose Orion’s main quadrangle, it is the star at the south-eastern corner. A northern-hemisphere observer facing south would see it at the lower left of Orion, and a southern-hemisphere observer facing north would see it at the upper right. The name Saiph is from the Arabic saif al jabbar, literally sword of the giant. This name was originally applied to Eta Orionis.

Parallax measurements yield an estimated distance of 650 light-years (198 parsecs) from Earth, which is about the same as Rigel. However despite being a hotter star, it is smaller and less luminous than Rigel with an apparent visual magnitude of 2.1. The luminosity of this star changes slightly, varying by 0.04 magnitudes.

Compared to the Sun this is an enormous star, with 14–17 times the mass and over 22 times the radius. It has a stellar classification of B0.5 Iab. The luminosity class ‘Iab:’ represents a supergiant star that has exhausted the supply of hydrogen at its core and evolved away from the main sequence. However, the ‘:’ indicates some uncertainty in the spectral value. Saiph has a strong stellar wind and is losing mass at the rate of 9.0 x 1E−7 times the mass of the Sun per year, or the equivalent of the Sun’s mass every 1.1 million years. Large stars such as Saiph (and many other stars in Orion) are destined to collapse on themselves and explode as Type II supernovae.


Betelgeuse, also known by its Bayer designation Alpha Orionis (shortened to α Orionis or α Ori), is the ninth-brightest star in the night sky and second-brightest in the constellation of Orion. Distinctly reddish, it is a semiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first-magnitude star. Betelgeuse is one of three stars that make up the Winter Triangle, and it marks the center of the Winter Hexagon. The star’s name is derived from the Arabic Ibt al-Jauzā’, meaning “the hand of Orion”. The Arabic letter for Y was misread as B by medieval translators, creating the initial B in Betelgeuse.

The star is classified as a red supergiant of spectral type M2Iab and is one of the largest and most luminous observable stars. If Betelgeuse were at the centre of the Solar System, its surface would extend past the asteroid belt, possibly to the orbit of Jupiter and beyond, wholly engulfing Mercury, Venus, Earth and Mars. Estimates of its mass range from 5 to 30 times that of the Sun. Its distance from Earth was estimated in 2008 at 640 light-years, yielding a mean absolute magnitude of about −6.02. Less than 10 million years old, Betelgeuse has evolved rapidly because of its high mass. Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion’s Belt—this crimson runaway has been observed moving through the interstellar medium at a supersonic speed of 30 km/s, creating a bow shock over 4 light-years wide. Currently in a late stage of stellar evolution, the supergiant is expected to proceed through its life cycle before exploding as a type II supernova within the next million years. An observation by the Herschel Space Observatory in January 2013 revealed that the star’s winds are crashing against the surrounding interstellar medium.


Credit: ESA/Herschel/PACS/L. Decin et al, Betelgeuse is the nearest red supergiant star to Earth

Below is an artist’s impression and shows how the bow shock structure is oriented with respect to Betelgeuse, the flow of the interstellar medium, and the Earth. A discontinuity in density and pressure appears at the boundary where stellar wind from Betelgeuse collides into interstellar matter. Betelgeuse moves in space from lower right to upper left in this figure.


The death of the Sun


The Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5.4 billion years and start to turn into a red giant.

1) Hydrogen is depleted in the core. Initially, the temperature in the core is not hot enough to ignite helium burning.

2) With no additional fuel in the core, fusion dies out. The core cannot support itself so it contracts and as it shrinks the temperature increases.

3) The rising temperature in the core heats up a thin shell around the core until the temperature reaches the point where hydrogen burning ignites in this shell around the core.


4) With the additional energy generation in the H-burning shell, the outer layers of the star expand but their temperature decreases as they get further away from the centre of energy generation. This large but cool star is now a red giant, with a surface temperature of 3500 degrees and a radius of about 100 solar radii.


5) The helium core contracts until its temperature reaches about 100 million degrees and the first red giant phase ends abruptly. Helium begins to fuse into carbon, providing a fresh new energy source.

6) The core cannot expand as much as required to compensate for the increased energy generation caused by the helium burning. Because the expansion does not compensate, the temperature stays very high, and the helium burning proceeds furiously. With no safety valve, the helium fusion is uncontrolled and a large amount of energy is suddenly produced. This helium flash occurs within a few hours after helium fusion begins.

7) The core explodes, the core temperature falls and the core contracts again, thereby heating up. When the helium burns now, however, the reactions are more controlled because the explosion has lowered the density enough. Helium nuclei fuse to form carbon, oxygen, etc.

8) The star wanders around the red giant region, developing its distinct layers, eventually forming a carbon-oxygen core.


9) When the helium in the core is entirely converted into C, O, etc., the core again contracts, and thus heats up again. In a star like the Sun, its temperature never reaches the 600 million degrees required for carbon burning. Instead, the outer layers of the star eventually become so cool that nuclei capture electrons to form neutral atoms (rather than nuclei and free electrons). When atoms are forming by capturing photons in this way, they cause photons to be emitted; these photons then are readily available for absorption by neighbouring atoms and eventually this causes the outer layers of the star to heat up. When they heat up, the outer layers expand further and cool, forming more atoms, and releasing more photons, leading to more expansion. In other words, this process feeds itself.

10) The outer envelope of the star blows off into space, exposing the hot, compressed remnant core. This is a planetary nebula.

11) The core contracts but carbon burning will never ignite in our low mass star, the Sun. Contraction is halted when the electrons become degenerate, that is when they can no longer be compressed further. The core remnant as a surface temperature of a hot 10,000 degrees and is now a white dwarf.

12) With neither nuclear fusion nor further gravitational collapse possible, energy generation ceases. The star steadily radiates is energy, cools and eventually fades from view, becoming a black dwarf.


Evolution of the Sun’s luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010)


The size of the current Sun (now in the main sequence) compared to its estimated size during its red-giant phase in the future


The above diagram shows the evolution of a Sun-like star. The track of a one solar mass star on the Hertzsprung–Russell diagram is shown from the main sequence to the post-AGB stage.

The asymptotic giant branch is the region of the Hertzsprung–Russell diagram populated by evolving low- to medium-mass stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives.

The death of large stars

Above about 8 solar mass (initial mass on the main sequence), massive stars will not be able to end their lives as quiet planetary nebula and white dwarfs. Massive stars are destined for a more spectacular finish.


Like all stars, high mass stars start out on the Main Sequence, fusing H to He in the core. As the core becomes full of He and the central star collapses, a shell H fusion to He begins and the star leaves the main sequence. High mass stars eventually evolve into the most luminous supergiant stars, first as Red Supergiants, then later moving back towards being Blue Supergiants.


Deep inside a massive supergiant star, advancing stages of more complex fusion is progressing. A high mass star can reach the needed high temperature and densities required to fuse all the way creating IRON at its core.

A massive star’s core burns furiously fast. As it fuses larger atoms together, it does so faster and faster, trying to maintain adequate pressure on the inside to offset the gravitational pressure pushing in from the outside. Below is the time scale over which a star 25 times the mass of the sun burns its internal fuel:



Each successive fusion reaction converts a smaller and smaller volume of core material, leaving an onion skin or shells of infused by-products from the previous fusion stage. Note that each consecutive stage is able to maintain the stars fuel supply for a shorter and shorter time.

Once the star’s central core is made up entirely of Iron, there is no more energy to be gained from fusion. This is because Iron is the most densely packed atom in the universe. With absolutely NO SOURCE OF FUEL LEFT, the central core and the rest of the star goes into a pure free fall as gravity finally takes over.

Production of planetary nebula

Stars greater than 8 solar masses (M⊙) will likely end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 M⊙ to 8.0 M⊙. Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their hydrogen into helium in the star’s core by nuclear fusion at about 15 million K. This generated energy creates outward pressure from fusion reactions in the core, equally balancing the crushing inward pressures of the star’s gravity. Hence, all single intermediate to low-mass stars on the main sequence can last for tens of millions to billions of years.

When the hydrogen source in the core starts to diminish, gravity starts compressing the core, causing a rise in temperature to about 100 million K. Such higher core temperatures then make the star’s cooler outer layers expand to create much larger red giant stars. This end phase causes a dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, even though the average surface temperature is lower. In stellar evolution terms, stars undergoing such increases in luminosity are known as asymptotic giant branch stars (AGB).

For the more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 3M⊙, their cores will continue to contract. When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen, so that the star again resumes radiating energy, temporarily stopping the core’s contraction. This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a short period compared to the entire lifetime of the star.

In either scenario, the venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula.

After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blown away from the central star at speeds of a few kilometres per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB. As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling once the hydrogen shell is exhausted through fusion and mass loss. In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the star.

Planetary nebulae play a very important role in galactic evolution. The early universe consisted almost entirely of hydrogen and helium, but stars create heavier elements via nuclear fusion. The gases of planetary nebulae thus contain a large proportion of elements such as carbon, nitrogen and oxygen, and as they expand and merge into the interstellar medium, they enrich it with these heavy elements, collectively known as metals by astronomers.

A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm^3.


NGC 6720, The Ring Nebula Credit: STScI/AURA


The Cat’s Eye Nebula: Credit: J.P. Harrington and K.J. Borkowski (U. Maryland), HST, NASA


Saturn Nebula NGC 7009 by Hubble Space Telescope


NGC 7293, The Helix Nebula – Credit: NASA, ESA, and C.R. O’Dell (Vanderbilt University).


Features of the Helix nebula

A Star and hence our Sun, is an almost entirely ionized ball of plasma, consisting of electrons and ions, in which there is hardly any gas (neutral atoms).


Megan is using a plasma ball to demonstrate the behaviour of a plasma. The plasma ball can actually cause the fluorescent tube light to light up (apologies for poor photography).


Megan explaining the path taken by a Sun like star as it leaves the main sequence


Stars greater than 8 solar masses explode in a supernova

A supernova is an explosion of a massive supergiant star. It may shine with the brightness of 10 billion suns! The total energy output may be 1 x E44 joules, as much as the total output of the sun during its 10 billion year lifetime. The likely scenario is that fusion proceeds to build up a core of iron. The “iron group” of elements around mass number A = 60 are the most tightly bound nuclei, so no more energy can be produced from nuclear fusion.

In fact, either the fission or fusion of iron group elements will absorb a dramatic amount of energy – like the film of a nuclear explosion run in reverse. If the temperature increase from gravitational collapse rises high enough to fuse iron, the almost instantaneous absorption of energy will cause a rapid collapse to reheat and restart the process. Out of control, the process can apparently occur on the order of seconds after a star lifetime of millions of years. Electrons and protons fuse into neutrons, sending out huge numbers of neutrinos. The outer layers will be opaque to neutrinos, so the neutrino shock wave will carry matter with it in a cataclysmic explosion.


The synthesis of the heavy elements is thought to occur in supernovae, that being the only mechanism which presents itself to explain the observed abundances of heavy elements.

The star becomes a neutron star it is less than 3 solar masses or a black hole if it is greater than 3 solar masses

A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star after a supernova. Neutron stars are the densest and smallest stars known to exist in the universe; with a radius of only about 12–13 km, they can have a mass of about two times that of the Sun.

Neutron stars are composed almost entirely of neutrons, which are subatomic particles without net electrical charge and with slightly larger mass than protons. Neutron stars are very hot and are supported against further collapse by quantum degeneracy pressure due to the phenomenon described by the Pauli exclusion principle, which states that no two neutrons (or any other fermionic particles) can occupy the same place and quantum state simultaneously.

A typical neutron star has a mass between ~1.4 and about 3 solar masses (M☉) with a surface temperature of ~6 x E5 K. Neutron stars have overall densities of 3.7 x E17 to 5.9 x E17 kg/m^3 (2.6 x E14 to 4.1 x E14 times the density of the Sun), which is comparable to the approximate density of an atomic nucleus of 3 x E17 kg/m^3. The neutron star’s density varies from below 1 x E9 kg/m^3 in the crust – increasing with depth – to above 6 x E17 or 8 x E17 kg/m^3 deeper inside (denser than an atomic nucleus). A normal-sized matchbox containing neutron star material would have a mass of approximately 5 billion tonnes or ~1 km^3 of Earth rock.

Some neutron stars rotate very rapidly (up to 716 times a second, or approximately 43,000 revolutions per minute) and emit beams of electromagnetic radiation as pulsars. Indeed, the discovery of pulsars in 1967 first suggested that neutron stars exist. Gamma-ray bursts may be produced from rapidly rotating, high-mass stars that collapse to form a neutron star, or from the merger of binary neutron stars. There are thought to be on the order of 108 neutron stars in the galaxy, but they can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Non-rotating and non-accreting neutron stars are virtually undetectable; however, the Hubble Space Telescope has observed one thermally radiating neutron star, called RX J185635-3754.


Radiation from the pulsar PSR B1509-58, a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, from Chandra) and illuminates the rest of the nebula, here seen in infrared (blue and red, from WISE).

A black hole is a mathematically defined region of spacetime exhibiting such a strong gravitational pull that no particle or electromagnetic radiation can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is general consensus that supermassive black holes exist in the centres of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. Such observations can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sgr A*, at the core of our own Milky Way galaxy, contains a supermassive black hole of about 4.3 million M☉.


Supernova SN 2001el was studied with the FORS1 spectropolarimeter mounted on ESO’s VLT-Melipal telescope.


The Crab Nebula photographed by Hubble


Tycho Supernova Remnant (NASA, Chandra, 03/24/11)


Credit: X-ray: NASA/CXC/Rutgers/K.Eriksen et al.; Optical: DSS

This image comes from a very deep Chandra observation of the Tycho supernova remnant, produced by the explosion of a white dwarf star in our Galaxy. Low-energy X-rays (red) in the image show expanding debris from the supernova explosion and high energy X-rays (blue) show the blast wave, a shell of extremely energetic electrons. These high-energy X-rays show a pattern of X-ray “stripes” never previously seen in a supernova remnant.

These stripes may provide the first direct evidence that supernova remnants can accelerate particles to energies a hundred times higher than achieved by the most powerful particle accelerator on Earth, the Large Hadron Collider. The results could explain how some of the extremely energetic particles bombarding the Earth, called cosmic rays, are produced, and they provide support for a theory about how magnetic fields can be dramatically amplified in such blast waves.

The X-ray stripes are thought to be regions where the turbulence is greater and the magnetic fields more tangled than surrounding areas. Electrons become trapped in these regions and emit X-rays as they spiral around the magnetic field lines. Regions with enhanced turbulence and magnetic fields were expected in supernova remnants, but the motion of the most energetic particles — mostly protons — was predicted to leave a messy network of holes and dense walls corresponding to weak and strong regions of magnetic fields, respectively. Therefore, the detection of stripes was a surprise.

The Tycho supernova remnant is named for the famous Danish astronomer Tycho Brahe, who reported observing the supernova in 1572. It is located in the Milky Way, about 13,000 light years from Earth. Because of its proximity and intrinsic brightness, the supernova was so bright that it could be seen during the daytime with the naked eye.

A stellar black hole (or stellar mass black hole) is a black hole formed by the gravitational collapse of a massive star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma ray burst. These black holes are also referred to as collapsars.

An Intermediate-mass black hole (IMBH) is a hypothetical class of black hole with mass in the range 100 to one million solar masses: significantly more than stellar black holes but less than supermassive black holes. There is as yet no unambiguous detection of an IMBH but the indirect evidence from various directions is tantalizing.

A supermassive black hole (SMBH) is the largest type of black hole, on the order of hundreds of thousands to billions of solar masses (M☉), and is found in the centre of almost all massive galaxies. In the case of the Milky Way, the SMBH is believed to correspond with the location of Sagittarius A*.

Supermassive black holes have properties that distinguish them from lower-mass classifications. First, the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass: a person on the surface of the Earth and one at the event horizon of a 10 million M☉ black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole.

The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light.

In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman’s terms, it is defined as “the point of no return”, i.e., the point at which the gravitational pull becomes so great as to make escape impossible. An event horizon is most commonly associated with black holes. Light emitted from beyond the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer’s side appears to slow down and never quite pass through the horizon,[citation needed] with its image becoming more and more redshifted as time elapses. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time. From here to the central singularity will take 0.0001 seconds in proper time, in free fall, for a 30 solar mass black hole. This infall time is proportional to the mass of the black hole.


Above right is a common graphical representation of a black hole. The diagram depicts the deepening gravity well that surrounds a black hole. The depth of the hole is infinite.

Since a black hole has no size, one never reaches the point where some mass is “behind” it so to speak. The object just keeps getting closer to the singularity where the well is deeper, with no limit.

One of the nasty effects an object experiences as it falls deeper and deeper into the gravity well is that across its own dimension (in the direction of the black hole) it experiences different strengths of gravity. The side closest to the black hole experiences a stronger acceleration than the side further away. It happens around the Earth too, but the effect is negligible.

As an object slides into the gravity well, the side nearest the well experiences more acceleration than the side further away.

The disparity on the pull of different parts of the object, depending upon their distance from the singularity, grows as the object falls further into the well. The result will be to distort the object and eventually “spigettify” it.


An image of what a black hole might look like

AGN outflows: feedback for galaxy evolution

An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.

Some observational characteristics:

X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present.

X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.

Strong absorption but normal high-E flux


AGN in NGC 5548 – X-ray spectra

Megan is explaining the results from some of the research she is involved with

In this plot of X-ray flux (the amount of X rays from the object that reaches us) vs energy, you can see the low energy (soft) X-rays on the left and the higher energy (hard) X-rays on the right. It is clear the the soft X-rays were much brighter in 2002 than we saw in 2013-2014, but the hard X-ray data from 2013-2014 joins up very nicely with the 2002 observations. The labels and colours on the graph refer to different space observatories (Chandra, NuSTAR, INTEGRAL) or particular instruments on space observatories (RGS and pn, both from XMM-Newton) that collected that range of data.


Supermassive black holes in the nuclei of active galaxies expel large amounts of matter through powerful winds of ionized gas. The archetypal active galaxy NGC 5548 has been studied for decades, and high-resolution X-ray and UV observations have previously shown a persistent ionized outflow. An observing campaign in 2013 with six space observatories shows the nucleus to be obscured by a long-lasting, clumpy stream of ionized gas never seen before. It blocks 90% of the soft X-ray emission and causes simultaneous deep, broad UV absorption troughs. The outflow velocities of this gas are up to five times faster than those in the persistent outflow, and at a distance of only a few light days from the nucleus, it may likely originate from the accretion disk.

Megan has been investigating the active galaxy NGC 5548, and analysing data taken from six space observatories (XMM-Newton, Hubble, Swift, NuSTAR, INTEGRAL and Chandra) as well as two ground based locations (in Israel and Chile).

“Active galaxies have a supermassive black hole in their centre, pulling in gas and dust from its surroundings. As the gas and dust falls towards and travels around the black hole, it heats up and emits radiation over the whole electromagnetic spectrum. As well as material falling in, some material can be pushed outwards from the black hole through this messy ‘feeding’ process. We intended to calculate distances from the central black hole of these outflows, called warm absorbers, previously observed in NGC 5548. The hard X-ray data indicated that the material around the central black hole was still emitting just as much light as it had been in the past and analysing a decrease in soft X-rays reaching us led us to conclude that a new stream of outflowing gas, which I will call the obscurer, has flowed into our line of sight (the direction we look at this system from Earth). This new stream is travelling at about 5000 km per second (over ten million miles per hour) and must be closer to the central black hole than the warm absorber outflows are. This is because the lower ionisation of the warm absorbers can be explained if the new obscurer is absorbing some of the light from around the black hole before it gets to the warm absorbers, so therefore the obscurer is closer to the source of this light (the material around the central black hole).”


The green line in this artist’s impression is the direction we are looking at NGC 5548 from Earth. The red area is very close to the supermassive black hole in the centre of NGC 5548 and the white stream close to that, and along the line of sight, represents the newly discovered obscurer. Credit: Renaud Person


The X-ray obscurer (stream of outflowing gas near the disk) has been continuously present for a few years.

The obscuring material is constantly being replenished from the accretion disk.

Changes in obscuration (covering fraction) produces the observed X-ray hardness ratio variability.

The colder phase (dense clumps) of the obscurer varies on a shorter timescale (days) than its warmer medium (months).

There is a direct link between UV and soft X-ray excess. Soft excess origin: optically thick, warm Comptonisation (the result of the Compton Effect; particularly, the redistribution of the energies of photons scattered by electrons in space).

The Compton Effect describes the fact that some energy and momentum is transferred to a stationary electron when a photon of sufficient energy and momentum collides with it.


Megan summarising the life cycle of low mass stars


Megan summarising the life cycle of high mass stars


Audience listening intently


Students standing in front of part of the planetarium


Sulaxan, Nicholas, Sarangan, Sujeethan and Abbas standing in front of The Weller Astronomy Galleries at the end of the study day

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