Year 13 Astronomy day at the Royal Greenwich Observatory

Astronomy day at the Royal Greenwich Observatory

The year 13 physics students were privileged to spend a day at the Royal Greenwich Observatory listening to some excellent lectures and taking part in an activity which allowed them to work out the approximate age of the universe.

The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. At this time the king also created the position of Astronomer Royal (initially filled by John Flamsteed), to serve as the director of the observatory and to “apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation.” The building was completed in the summer of 1676. The original building was often given the title “Flamsteed House”.

Most of the activities took place in the more recent Astronomy centre. .

Find out more from

The first activity of the day was a lecture given by Dr Radmilla Topalovic on stellar evolution. See

The lecture began with an introduction to the Hertzsprung-Russell diagram.

The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars’ absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars. Rather, they plot each star on a graph measuring the star’s absolute magnitude or brightness against its temperature and colour. Blue stars are very hot, red stars are very cold and yellow stars like are Sun have an intermediate temperature.

The stars are placed into groups O, B, A, F, G, K, M. O is the hottest and M is the coldest. Our Sun is a G star.

The main sequence stars are fully grown, stable stars but they leave the main sequence when their properties change.

Stars begin their lives in nebulae. A cold gas starts to contract due to the influence of gravity. As it contracts it heats up as gravitational potential energy is converted into thermal kinetic energy. At a certain point a protostar forms (a baby star). As matter clumps together the spin of the protostar increases. Any leftover matter becomes planets. Infra-red radiation can be used to view protostars through the dust clouds.

We looked at some beautiful images of some nebulae.

Emmission nebula NGC 2024 known as the flame nebula can be found in the constellation Orion. It is about 900 to 1500 light years away with a temperature of about 10000K.

The horsehead nebula is a dark nebula also found in the constellation Orion. It is about 1500 light years away with a temperature of between 10 and 100K.

NGC 2023 is a reflection nebula. It doesn’t emit any light of its own. It just reflects light.


The left picture is the “spire” within the Eagle nebula the picture on the right is a picture of the “pillars of creation” region of the Eagle nebula. The small dark areas are believed to be protostars. The Eagle nebula can be found in the constellation Serpens and it is about 7000 light years away.

The Orion nebula hides a large region bursting with immature stars. It is chaotic, seriously overcrowded and not very well run (unlike most stellar nurseries).

The Herschel telescope has a 3.5m mirror and is situated 1.5 million km from Earth.

As of 2012 it is the largest infrared telescope ever launched. It is capable of seeing the coldest and dustiest objects in space. As it is in cold space it is not affected by the atmosphere or any infrared on the Earth.

Herschel specialises in collecting light from objects in our Solar System as well as the Milky Way and even extragalactic objects billions of light-years away, such as newborn galaxies, and is charged with four primary areas of investigation:

NGC 1999 (a reflection nebula) as seen by the Herschel telescope (the green tinged region). Distance of 1000 light years from the Earth.

ESA’s Herschel infrared space telescope made an unexpected discovery: a vast hole in space. The hole has provided astronomers with a surprising glimpse into the end of the star-forming process.

A cloud of bright reflective gas known to astronomers as NGC 1999 sits next to a black patch of sky. For most of the 20th century, such black patches have been known to be dense clouds of dust and gas that block light from passing through.

When Herschel looked in its direction to study nearby young stars, the cloud continued to look black. But Herschel’s infrared eyes are designed to see into such clouds. Either the cloud was immensely dense or something was wrong.

Investigating further using ground-based telescopes, astronomers found the same story however they looked: this patch looks black not because it is a dense pocket of gas but because it is truly empty. Something has blown a hole right through the cloud. “No-one has ever seen a hole like this,” says Tom Megeath, of the University of Toledo, USA. “It’s as surprising as knowing you have worms tunnelling under your lawn, but finding one morning that they have created a huge, yawning pit.”

The astronomers think that the hole must have been opened when the narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas that forms NGC 1999. The powerful radiation from a nearby mature star may also have helped to clear the hole. Whatever the precise chain of events, it could be an important glimpse into the way newborn stars disperse their birth clouds. See  

Star formation can be triggered by two galaxies colliding. Our galaxy will eventually collide with Andromeda.                                                                                                                                                        As the core of a protostar collapses hydrogen fusion begins when the temperature rises to 1×107K in the centre. See

4H –>He + energy

4.01996u = 4.00255u + 0.017u (energy) Application of the famous equation E = m(c)sq

u = unified atomic mass unit (1.660538921(73)×E−27 kg).

It is so hot inside the core that electrons are free from the hydrogen.

In our Sun there are 1xE38 reactions per second. Its luminosity is 3.8xE26W. The 0.017u = 4.36746 x E-29kg and the energy released during each fusion using E = mc2 is 3.93xE-12J.

We then turned our attention to main sequence stars. See

Mass determines the lifetime of a star. Most massive star is R136a1.

R136a1 is a blue hypergiant. Its surface temperature is 40000K and is 265 solar masses. It is also the brightest with a luminosity of 8,700,000 times that of the Sun. See

Biggest star is Vy Canis Majoris with a radius of between 1800 to 2100 that of the Sun. It is a red hypergiant with a surface temperature of 30000K.


Main sequence stars have masses between 0.08 and 100 solar masses. If the mass is below 0.08 solar masses then the star fails to start hydrogen fusion and becomes a brown star.

The Sun isn’t actually near Rigel. It allows you to compare sizes. Rigel is 17 times the size of our Sun. Its surface temperature is about 10000K. See

We then looked at what happens when a star leaves the main sequence and dies.

1) Hydrogen becomes depleted in the core.

2) The core contracts and the temperature increases.

3) Temperature reaches 1xE8K in the core and helium starts to burn. See

If the star is about the same size as our Sun then when it runs out of hydrogen in its core there is no longer any source of heat to support the core against gravity. The core of the star collapses under gravity’s pull until it reaches a high enough density to start burning helium (helium flash) to carbon. The star’s outer envelope expands and the star evolves into a red giant. Eventually all the mass in the envelope is lost leaving behind a hot core of carbon. Once this core cools the star becomes a white dwarf.

If the star is a lot bigger than our Sun then the helium flash does not occur. It burns brighter and dies more dramatically. If it is about 25 times bigger running out of helium does not stop the nuclear burning cycle. The carbon core contracts further and reaches a high enough temperature to burn carbon to oxygen, neon, silicon, sulphur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to a heavier element.

If the star has a mass less than 8 solar masses then the dying star becomes a planetary nebula. The helium flash sends thermal pulses through the star. Outer layers of the star become detached and extensive mass loss occurs.

Hubble image of NGC6543 Planetary nebula also known as the Cat’s eye nebula.

Butterfly nebula. 3,800 light years away in the constellation scorpius. It is about 2 light years across and the “wings” are made of gas heated to more than 36000 degrees Celsius with a dying star at the centre. The Hubble telescope used special filters to isolate the light of different elements. Pink is ionised nitrogen and blue is ionised oxygen.

If the star has a mass greater than 8 solar masses then it becomes a supernova. Without any source of heat to balance the gravity, the iron core collapses rapidly. This high density core resists further collapse causing the infalling matter to “bounce” off the core. This sudden core bounce (which includes the release of energetic neutrinos and neutrons from the core owing to electrons and protons combining) produces a supernova explosion (inner regions expand creating a pressure wave). The outward motion of matter exceeds the speed of sound creating a shock wave. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into future generations of stars and planets. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.

The crab nebula is a supernova remnant in the constellation of Taurus. See

It was spotted in July 1054 AD and it is about 6000 light years away.

Pictured above is the best multi-wavelength image yet of Tycho’s supernova remnant, the result of a stellar explosion first recorded over 400 years ago by the famous astronomer Tycho Brahe. The above image is a composite of an X-ray image taken by the orbiting Chandra X-ray Observatory, an infrared image taken by the orbiting Spitzer Space Telescope, and an optical image taken by the 3.5-meter Calar Alto telescope located in southern Spain. The expanding gas cloud is extremely hot, while slightly different expansion speeds have given the cloud a puffy appearance. Although the star that created SN 1572, is likely completely gone, a star dubbed Tycho G, too dim to be easily discerned here, is being studied as the possible companion. Finding progenitor remnants of Tycho’s supernova is particularly important because the supernova was recently determined to be of Type Ia. The peak brightness of Type Ia supernovas is thought to be well understood, making them quite valuable in calibrating how our universe dims distant objects.

If the remains of the star after the supernova explosion has a mass of between 1.3 and 3 solar masses then a pulsar can form. It is incredibly dense with a strong magnetic field. The star rotates very quickly and high energy particle sweep around because of it. See

A neutron star has a mass of between 1.35 and 2 solar masses. See

Neutron stars are so dense that you can think of them as being the equivalent of the whole human race compressed to the size of a sugar cube.

If the remains of the star has a solar mass greater than 3 solar masses then a black hole forms as there are no known nuclear forces to prevent the core from forming a deep gravitational warp in space. See
Chandrasekhar in 1928 calculated the limit whereby a cold star cannot maintain its balance in terms of a constant radius, between its gravity and the “exclusion principle” which makes the star want to expand.
Stars exceeding the Chandrasekhar limit, those which have a mass of over twice that of our sun, cannot settle into the stable state of a white dwarf. They may cool and swell to become a red giant, more massive stars may become supergiants the most massive exploding as supernova.
In some cases these outcomes may occur with stars exploding thus throwing off sufficient matter to avoid gravitational collapse. Another scenario is that the star collapses under its own gravity.
With such a star exhausted of nuclear fuel nothing remains of the outward pressure supporting the star against its own gravity. As the star shrinks the gravitational field becomes stronger. A point is reached where the escape velocity necessary to overcome the gravitational field cannot be exceeded even by light itself at a speed of 300,000 kilometres/second. At this point in time light is no longer emitted from the star, a Black Hole has been formed.
Black Holes have a boundary the point at which light just fails to escape, this is called the Event Horizon or Schwarzschild radius which can be calculated from the formula:


                Where:       G = Newton's Constant of gravity
                            M = Mass of star
                            C = Velocity of light

For example: for a star with a mass of 19.9 x E30Kg (A mass of about 10 times that of our Sun) this calculates to give a Schwarzschild radius of about 30 kilometres.
There are a number of methods which have been proposed to assist in locating black holes. One such method takes into account that a black hole, whilst emitting no light, will still exert a gravitational force.
Observations have been made in systems of two stars orbiting each other, being attracted by their respective gravitational forces. Other observations have been made where a star appears to orbit an unseen partner.
A good example of this effect is Cygnus X-1.

There are three classes of black holes:

Stellar – These have masses between 5 and 10 solar masses.

Intermediate – These have masses between 100 and 1000 solar masses.

Supermassive – These have masses between millions and billions solar masses. Our galaxy is believed to have one at its centre.

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