Once again Rooks Heath year 13 physics students were lucky to take part in an astronomy masterclass at Royal Greenwich Observatory. This year we were able to share the day with students from Harrow High School and Cannons High School.
Sulaxan, Abbas, Nicholas and Sujeethan enjoying the view before the classes start.
Abbas, Nicholas, Sarangan, Sulaxan and Sujeethan with historic Greenwich behind them
The first class of the day was Studying Starlight with Dr Radmilla Topalovic
Radmilla started the session by revising work that the students had covered at GCSE and AS.
Sujeethan, Sulaxan, Sarangan, Nicholas and Abbas listening to the introduction to the activity
Radmilla reminding the students about how a transverse can be drawn
Waves are disturbances that transfer energy, not matter. Light waves can propagate through a vacuum unlike sound waves.
A transverse wave is a wave which has its vibration directions at right angles to the propagation direction.
The amplitude (the maximum displacement from the equilibrium position), also known as the height of a peak or the depth of a trough, is an indication of the energy carried by the wave
If the graph is of displacement against propagation direction the distance between two neighbouring points in phase (shown above as the distance between two neighbouring peaks) is the wavelength l (unit = metre)
If the graph had been displacement against time then the distance between two neighbouring points in phase is the time period (T). This is the time for one complete oscillation to occur (unit = second)
The frequency of the wave is the number of oscillations/waves passing a point per second (unit is the Hertz or Hz). It equals 1/T
Radmilla reminding the students about the electromagnetic spectrum
The electromagnetic spectrum is a special group of transverse waves that do not need a medium to travel through and move at a speed of 3xE8 m/s in a vacuum.
In some respect the divisions within the electromagnetic spectrum are a bit vague because the spectrum is continuous. Each type of wave runs into the next so radio waves run into microwaves, which in turn run into Infra-red etc. There is in fact an overlap between X- rays and gamma rays and the way to distinguish them is to consider how they are produced.
Each part of the electromagnetic spectrum is categorised by its wavelength/frequency which in turn dictates the amount of energy the wave carries. Gamma rays are the most energetic and radio waves are the least.
Blue light (frequency = 7.5 x E14): 3 x E8 ÷ 7.5 x E14 = 400 x E19 m = 400 nm (nm = nanometre)
Red light (frequency = 4.3 x E14): 3 x E8 ÷ 4.3 x E14 = 698 x E19 m = 698 nm
X‐ray (frequency = 3.0 x E17): 3 x E8 ÷ 3.0 x E17 = 1 x E19 m = 1 nm
Radio wave (frequency = 3.0 x E5): 3 x E8 ÷ 3.0 x E5 = 1000 m = 1 km
Electromagnetic waves are actually made up of two types of wave oscillating at 90 degrees to each and both of these also oscillate at 90 degrees to the propagation direction of the wave.
The electromagnetic waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized EMR wave (only one oscillation direction present) propagating from left to right. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The electric and magnetic fields in EMR waves are always in phase and at 90 degrees to each other.
Electromagnetic waves are produced whenever charged particles are accelerated, and these waves can subsequently interact with any charged particles. EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact.
James Clerk Maxwell first formally postulated electromagnetic waves. These were subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave.
According to Maxwell’s equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, and vice versa. This relationship between the two occurs without either type field causing the other; rather, they occur together in the same way that time and space changes occur together and are interlinked in special relativity. In fact, magnetic fields may be viewed as relativistic distortions of electric fields, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again affect the source. The distant EM field formed in this way by the acceleration of a charge carries energy with it that “radiates” away through space, hence the term.
We tend to concentrate on the electric field but there is, of course, the magnetic field. Remember the electrical field is perpendicular to the magnetic field but they both have the same speed through a vacuum (= 3 x E8 m/s)
All electromagnetic waves can be reflected, refracted, diffracted and polarised
As the Universe cooled after the Big Bang stars were formed from the hydrogen gas and fusion reactions in stellar cores produced helium and lots of other heavier elements. Large stars explode into a supernova and eject these elements into space to be recycled.
Stars are objects that produce enough energy (via nuclear fusion in their core) to maintain their size against gravitational collapse.
Stars, much bigger than our Sun, produced heavier elements in their core and delivered them to their surroundings when they went supernova. The ejected gas clumped together over time to form our Solar System – all of the elements we have on Earth come from a star.
Stars give out electromagnetic radiation and the type and quantity give us important information about those stars such as temperature, pressure, abundance of elements and motion (velocity)
The part of the electromagnetic spectrum that we are most used to is visible because we can see it but other parts are useful too.
Radmila talking about Andromeda
The Andromeda Galaxy (is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years; 2.4 x E19 km) from Earth. Also known as Messier 31, M31, or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest major galaxy to the Milky Way, but not the nearest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies.
The Spitzer Space Telescope revealed that M31 contains one trillion (1 x E12) stars: at least twice the number of stars in the Milky Way, which is estimated to be 200–400 billion. It is s estimated to be 1.5 x E12 solar masses.
At 3.4, the apparent magnitude of the Andromeda Galaxy is one of the brightest of any Messier objects,[ making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.
The images below are both of Andromeda. On the left is the visible light image and on the right is an ultraviolet image.
Ultraviolet astronomy is the observation of electromagnetic radiation at ultraviolet wavelengths between approximately 10 and 320 nanometres; shorter wavelengths—higher energy photons—are studied by X-ray astronomy and gamma ray astronomy. Light at these wavelengths is absorbed by the Earth’s atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.
The atmosphere also distorts light reaching us from space, light pollution reduces our ability to observe faint objects, and weather (clouds) affects our view of the night sky.
Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of the interstellar medium, and the temperature and composition of hot young stars. UV observations can also provide essential information about the evolution of galaxies.
The ultraviolet Universe looks quite different from the familiar stars and galaxies seen in visible light. Most stars are actually relatively cool objects emitting much of their electromagnetic radiation in the visible or near-infrared part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. If we could see the sky in ultraviolet light, most stars would fade in prominence. We would see some very young massive stars and some very old stars and galaxies, growing hotter and producing higher-energy radiation near their birth or death. Clouds of gas and dust would block our vision in many directions along the Milky Way.
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second. It has been measured approaching relative to our Sun at around 300 kilometres per second as the Sun orbits around the centre of our galaxy at a speed of approximately 225 kilometres per second. This makes Andromeda one of the few blueshifted galaxies that we observe. Andromeda’s tangential or side-ways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to directly collide with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or perhaps even a large disk galaxy. Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join M31.
So different parts of the electromagnetic spectrum gives use different information about stars and galaxies.
The peak wavelength of a star’s overall spectrum determines the colour of the star. Shorter wavelengths mean higher energy and bluer stars, longer wavelengths represent lower energy and the star will be redder.
In the mid-1660s Newton conducted experiments on light at Cambridge. He allowed a beam of sunlight to pass through a glass prism and saw the spectrum of visible light. Different colours have different wavelengths and they are the visible part of the electromagnetic spectrum. He published his results in a paper called The Opticks.
Illustration of a dispersive prism decomposing white light into the colours of the spectrum, as discovered by Newton
Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.
In 1666, Newton observed that the spectrum of colours exiting a prism in the position of minimum deviation is oblong, even when the light ray entering the prism is circular, which is to say, the prism refracts different colours by different angles. This led him to conclude that colour is a property intrinsic to light—a point which had been debated in prior years.
From 1670 to 1672, Newton lectured on optics. During this period he investigated the refraction of light, demonstrating that the multi-coloured spectrum produced by a prism could be recomposed into white light by a lens and a second prism. Modern scholarship has revealed that Newton’s analysis and resynthesis of white light owes a debt to corpuscular alchemy.
He also showed that coloured light does not change its properties by separating out a coloured beam and shining it on various objects. Newton noted that regardless of whether it was reflected, scattered, or transmitted, it remained the same colour. Thus, he observed that colour is the result of objects interacting with already-coloured light rather than objects generating the colour themselves. This is known as Newton’s theory of colour.
Radmilla talking about Fraunhofer lines
In 1814 in Germany, Josef Von Fraunhofer discovered dark lines in the Sun’s spectrum. These gaps are caused by the absorption of specific wavelengths of light by atoms in the Sun. Measuring the wavelengths of the spectral lines reveals the chemical composition of the Sun, which is 74% hydrogen, 24% helium and 1% heavier elements.
Joseph Fraunhofer (6 March 1787 – 7 June 1826), ennobled in 1824 as Ritter von Fraunhofer, was a German optician.
He is known for discovering the dark absorption lines known as Fraunhofer lines in the Sun’s spectrum, and for making excellent optical glass and achromatic telescope objectives.
In 1814, Fraunhofer invented the spectroscope. In the course of his experiments he discovered the bright fixed line which appears in the orange colour of the spectrum when it is produced by the light of fire. This line enabled him afterward to determine the absolute power of refraction in different substances. Experiments to ascertain whether the solar spectrum contained the same bright line in the orange as that produced by the light of fire led him to the discovery of 574 dark fixed lines in the solar spectrum; millions of such fixed absorption lines are now known.
These dark fixed lines were later shown to be atomic absorption lines, as explained by Kirchhoff and Bunsen in 1859.These lines are still called Fraunhofer lines in his honour; his discovery had gone far beyond the half-dozen apparent divisions in the solar spectrum that had previously been noted by Wollaston in 1802.
Fraunhofer also developed a diffraction grating in 1821, which occurred after James Gregory discovered the principles of diffraction grating and after American astronomer David Rittenhouse invented the first man-made diffraction grating in 1785. Fraunhofer found out that the spectra of Sirius and other first-magnitude stars differed from the sun and from each other, thus founding stellar spectroscopy.
Spectroscopy is an important tool to study physical and chemical properties of stars. Composition is one, motion is another and this becomes apparent through the Doppler Effect. This effect is best described by taking the example of a police car with its siren on, moving at high speed towards an observer. The crests of the sound wave will be closer together as the source moves towards the observer and therefore the frequency of the sound wave will be higher. As the police car moves away from the observer, the crests of the sound wave are spaced further apart and the frequency is lower. The same thing happens with light. If a star is moving slightly closer to us, the light is shifted to a slightly higher frequency. When the star moves slightly further away, the light is shifted to a slightly lower frequency.
Radmilla explaining how the spectrum of light from a star can tell you what the atar is made of. Our Sun is made up of Hydrogen (74%), helium (25%), lithium, calcium, magnesium, sodium, iron (these make up 1% of the Sun).
The Gemini Observatory is an astronomical observatory consisting of two 8.19-metre telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies. They are currently among the largest and most advanced optical/infrared telescopes available to astronomers. Gemini North can take images of spectra and stars.
Both Gemini telescopes employ a range of advanced technologies to deliver the highest quality images, including laser guide stars, adaptive optics, multi conjugate adaptive optics, and multi-object spectroscopy. In addition, the two telescopes allow very high-quality infrared observations due to the advanced protected silver coating of their mirrors, their small secondary mirrors (due to the f16 focal ratio), and advanced ventilation systems. Thanks to a high degree of networking, the Gemini telescopes can be operated remotely, and observations can be run when atmospheric conditions suit them best, reducing unnecessary travel by astronomers.
Each Gemini telescope is equipped with its own version of GMOS, which can perform multi object spectroscopy, long-slit spectroscopy, imaging, and integral field spectroscopy at optical wavelengths. Each of the 2 GMOS instruments are currently having their detectors upgraded to Hamamatsu arrays, which will have significantly improved performance in the far red part of the optical spectrum (700 nm – 1,000 nm).
Gemini’s silver coating and infrared optimisation allow sensitive observations in the mid infrared part of the spectrum (5-27 µm). This is covered by T-ReCS at Gemini South, and by Michelle at Gemini North, both of which have imaging and spectroscopic cababilities.
Gemini North has NIFS (Near-infrared Integral Field Spectrograph), built by Australian National University. It takes a 30 × 30 array of spectra (covering a three-arcsecond-square field) at R = 5000 in the 0.95-2.4 µm wavelength range.
Astronomers use diffraction gratings to split light into its constituent colours (spread out the light and provide dispersion). 12000 grooves per mm give high resolution spectroscopy.
100 lines per mm
600 lines per mm
The more lines there are the bigger the spread of the pattern
Revising energy levels
Electrons can only have discrete energies. This can be pictured as if the electrons are climbing a ladder. Each rung of the ladder is an energy level. If the electrons have the least energy they will sit on the lowest rung designated energy level n = 1, which is the ground state.
To move from one level to the next requires set amounts of energy. Photons with these amounts of energy are the ones absorbed by the gas. Other photons, with more or less energy than these values, are left untouched. Hence you get lines (which are gaps) on the spectrum where photons are missing.
We draw energy level diagrams like this:
Now it is very important that you realise that electrons don’t actually climb up and down a ladder. This is just a convenient way of imagining what is going on
If each level has an energy value, it is easy to see that the difference between the energy levels must equal the energy delivered by the photon.
hf = E1 – E2
E1 = the energy needed to be at level 1
E2 = the energy needed to be at level 2.
Difference between energy levels = E1 – E2
Note that there is an alternative method to exciting electrons. If you hit them with other electrons, they can gain just the right amount of energy from the collision to make the jump!
Why are the energy levels given negative values?
The concept here is repeated in the sections on gravitational and electric fields.
Consider an electron and the nucleus of an atom. They attract each other due to their opposite charge. As a result of this attraction the electron has potential energy, Ep.
As the electron moves away from the nucleus, this potential energy increases. However, as the electron moves away from the nucleus, the increased distance makes the force due to the electrical attraction smaller. At infinity, the attraction between the two is zero. No attraction – no potential energy.
So at the biggest distance from the nucleus, where you would logically expect to have the biggest value of potential energy, you find that the Ep value is in fact zero. That means that the biggest value possible is zero. Therefore, as you move back towards the nucleus and lose potential energy, you must be going below zero potential energy, i.e. you must have negative potential energy.
For this reason, all the electron energy levels in diagrams are given a negative value, showing that the potential energy at that point is less than it is at infinity.
So what do the electrons do once they are excited?
Electrons don’t really like being excited. Consequently, very shortly after becoming excited they drop back to the lower energy state. Of course that means that they have to lose some energy and to do this they give out a packet of energy – a photon. The size of the photon equals the size of the jump that they have to make. So once again you can calculate the expected frequency of the emitted photon using
hf = E1- E2
The more energetic the jumps (up or down) the higher the frequency of the photon absorbed or emitted. This means he further into the U-V end of the spectrum the lines go. The photon has a particular wavelength or colour and we see this as a brightly coloured line in the atomic emission spectrum
Absorption and emission spectra
Because each atom has a different electron structure, each element can be identified by the photons that it absorbs or emits from its electrons moving from energy level to energy level.
Single atoms give out the clearest results when you examine the spectra for different elements. That’s because when they are involved in bonding with other atoms, the electrons feel influences from places other than just the atom itself. That distorts the results. So to get the best results we use gas at low pressure in vapour tubes.
The absorption spectrum
Shine white light through vapour of a particular element. Analyse the spectrum of the light that has passed through the vapour. It has gaps in what is otherwise a perfect spectrum. That’s an absorption spectrum.
The emission spectrum
Excite electrons in a vapour. (Remember that you can do this in two ways. Either use photons or use other electrons.) Collect the photons that are emitted when the electrons drop from the higher energy levels down to the lower levels.
This will produce a series of coloured lines on a mainly black background – showing that light is only emitted at certain frequencies relating to certain jumps.
That’s an emission spectrum.
So the spectrum, whether absorption or emission, is a fingerprint that will identify the element (or elements in a compound)
The whole electromagnetic spectrum is used to study objects in space. Some parts of the spectrum such as X-rays and gamma rays are blocked by the atmosphere; to see this emission space telescopes are used. Black holes in binary systems (orbiting other stars) and the explosions of distant hypermassive stars have been detected using Xray and gamma ray telescopes. Observations of radio emission from cold hydrogen gas in the Milky Way in the 1950s revealed the structure of our galaxy and the presence of dark matter and infrared observations taken by terrestrial and space telescopes probe the dusty disks of protostars in star forming regions such as the Orion Nebula.
The spectrum of a protostar will show strong emission in the infrared due to the dusty circumstellar disk and nebula from which the star is formed. These lines will be quite broad due to the presence of molecules. A main sequence star will show strong hydrogen emission and helium, it may also show ionised atomic lines or lines from neutral heavy elements depending on the temperature of the star. A planetary nebula contains hot ionised gas so sharp emission lines will be seen in the UV and optical range. However in the cooler outer regions there may be emission from neutral atoms and small molecules at redder wavelengths and into the infrared. A white dwarf loses its outer nebula and glows at temperatures of up to 100 000 K so its spectrum will show strong emission lines from ionised atoms.
Activity 1: Match that spectra
The aim of this activity was to take a given spectra and match it to a picture of an object. The first part was relatively easy as they were given a pictures of raspberries, leaves and carrots and they knew that the raspberries would show an absorption spectra with red wavelengths present (a red object appears red because it will absorb most wavelengths except red), the leaves with green wavelengths present (a green object appears green because it will absorb most wavelengths except green) and they worked out that orange carrots would have red and yellow present (to give orange) with all the other colours being absent. They also learnt that beta carotene (the pigment that makes carrots orange) strongly absorbs UV light.
Sujeethan and Sulaxan starting the activity
Nicholas closely examining the absorption spectrum for green
The finished activity
The next activity involved using their knowledge to identify different star types
Sujeethan, Sulaxan and Sarangan working it out
Radmilla going through the answers
The green leaves on the left and the carrots on the right
Nicholas and Abbas got them all correct very quickly
Radmilla pointing out what the actual absorption spectra from the star looks like
Activity 2: What is in a star?
The gases were neon, helium and argon
Nicholas looked at the different spectra with a hand held spectrometer
Using a spectroscope and discharge lamps to investigate gases
Emission lines obtained from our Sun
Our Sun actually spins on its axis so the light we receive shows red and blue shift
Redshift and blueshift
Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).
In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, “redder” means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light.
The final activity of the session involved the students trying to decide if a star’s spectra show red shirt or blue shift.
If spectral lines are shifted towards the red end (longer wavelengths) relative to a spectrum at rest than that part of the Sun is moving away from us; a blue shift tells us that region of the Sun is approaching us. The extent of the shift tells us the velocity.
From the amount of red shift or blue shift you can calculate the rotational velocity.
Below is the visible (flattened) spectrum of the Sun taken by the McMath‐Pierce Solar Telescope on Kio Peak in Arizona. The Fraunhofer (absorption) lines can be seen and their relative intensities are shown. The Sun’s rotational velocity can be calculated by using its spectrum. Stars like the Sun often show strong hydrogen emission lines such as Hα, Hβ and Hγ (below). By measuring the wavelengths of these observed lines and comparing them to their rest wavelengths we can tell whether the part of the Sun we are looking at is redshifted or blueshifted and then we can calculate the velocity of the galaxy using the Doppler equation.
The angstrom is a unit of length equal to 1 x E−10 m (one ten-billionth of a metre) or 0.1 nm. Its symbol is Å, a letter in the Scandinavian alphabets.
The students looked at spectra to decide if there had been a blue or a red shift
They used the above equation to work out the rotational speed of the stars
V is the rotational speed, C is the speed of light in a vacuum, l is the measure wavelength and lo is the rest wavelength as measure in the laboratory