Once again Rooks Heath students were lucky enough to take part in an astronomy day at ROG.
The students were lucky to spend the morning just investigating the observatory and the information galleries.
The view for the observatory shows the Queen’s house and the Royal Hospital. The hospital was designed deliberately so as not to spoil the view of the Thames from the Queen’s House.
The 17th-century Queen’s House represents a turning point in English architecture. It was originally the home of Charles I’s queen, Henrietta Maria.
Greenwich Hospital was established in 1694 by Royal Charter for the relief and support of seamen and their dependants and for the improvement of navigation. The University of Greenwich has taken up residence in two of the four great Courts of the hospital.
Pameer taking the above picture
Matthew, Pameer, Wing Chun and Aslam
Matthew, Pameer, Aslam and Wing Chung standing on both sides of the Earth (the Greenwich Meridian)
The bright red Time Ball on top of Flamsteed House is one of the world’s earliest public time signals, distributing time to ships on the Thames and many Londoners. It was first used in 1833 and still operates today.
In the Time galleries
The picture below shows a globe with the times at the different longitudes. It was 10:59:30 at Greenwich
John Harrison (3 April 1693– 24 March 1776) was a self-educated English carpenter and clockmaker. He invented the marine chronometer, a long-sought after device for solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long-distance sea travel in the Age of Sail. The problem was considered so intractable, and following the Scilly naval disaster of 1707 so important, that the British Parliament offered the Longitude prize of £20,000 (comparable to £2.66 million/$4.25 million US in modern currency) for the solution.
Having a rest before moving on to the Weller astronomy galleries
The Royal Observatory South Building
Wing Chung and Pameer using one of the interactive exhibits
In the picture below Matthew is investigating spectra. You can gain a lot of information from looking at the light coming from stars such as their temperature, size and chemical composition.
Aslam (left) and Matthew (right) investigating the infra-red radiation coming from their hands
In the picture below Aslam is investigating starlight
Wing Chung reading about how astronomers use light
Wing Chung using one of the interactive exhibits to investigate the lifecycle of stars
The expanding Universe
In the picture above Matthew, Aslam and Pameer are listening to an introduction to the activity. In the session they were introduced to the Citizen online project Galaxy Zoo, whereby members of the public can classify galaxies and contribute to scientific research.
The term galaxy is a rather recent term and it would have been very new to Edwin Hubble in the 1920s. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936 Hubble produced a classification system for a galaxy that is used to this day, the Hubble sequence.
Cepheid variables are used to calculate distances up to a distance of 650 light years as they are standard candles. A standard candle is a star whose luminosity is known.
In Hubble’s time all telescopes (refractors and reflectors) were placed on hills to get the best images. The first galaxy (apart from our milky way) to be identified was Andromeda as it is the nearest to us.
The students were given real data on galaxies from the Sloan Digital Sky Survey and plotted a graph of velocity vs. distance. This enabled them to come up with a value for the Hubble constant as Hubble’s law states that v = Hod, where v is the velocity and d is the distant. They used basic equations to determine large-scale properties of the Universe and investigated the statistical significance of their results.
We know the Universe is expanding because when we look at the light from galaxies we know they are moving away from us and each other, accelerating with distance. Edwin Hubble first discovered this in 1923 using his 100” reflector on Mount Wilson in the US.
The problems and constraints with trying to measure the motion of the galaxies are due to the facts that very distant galaxies are faint and difficult to measure and the telescope must have a high sensitivity but also be capable of resolving emission spectra from galaxies. The atmosphere also affects observations and so a space telescope is useful for these kinds of studies but they can’t be as big as terrestrial telescopes.
To measure the distance to a galaxy, measure its intensity and find its luminosity. It is then a simple calculation to get the distance. Use its spectrum to find its velocity from Doppler shifted lines.
Telescopes unfortunately don’t pick up all the radiation coming from a star as energy gets spread out in all directions (forming an area of a sphere 4pr^2)
Radiative flux, also known as radiative flux density, radiation flux or intensity is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m2. The flux/intensity we measure on Earth equals L/4pr^2 where L is the luminosity of the star/galaxy and r is the distance between us and the galaxy. This means that if we know the luminosity of the star and we measure the flux of the galaxy we can calculate the distance.
The Doppler Effect is also made use of but in this case it isn’t about sound waves from moving objects but light shifts due to galaxies moving. Blue shift means the galaxy is moving towards us and red shift means the galaxy is moving away from us.
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.
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).
Z is a measure of the amount of red shift or blue shift and can be used to find the velocity (v) of the galaxy towards or away (recessional) from us. It is calculated by measuring the light wavelength received form the distant galaxy (l) and using the actual wavelength measured from the same light in the laboratory (lo).
Hubble’s law states that the recessional velocity of galaxies increases with their distance from us; this is shown in the graph below. The gradient is deﬁned as the Hubble constant, H0 which is usually given the units km s^‐1 Mpc^‐1, where Mpc is megaparsec, an astronomical unit equivalent to 3.09 x 10^19 km.
The distance of a galaxy can be determined from its measured intensity and intrinsic luminosity. Its velocity can be calculated by using its spectrum. A galaxy spectrum often shows strong hydrogen emission lines such as Hα (below). By measuring the wavelength of this observed line and comparing it to the rest wavelength (the value in the laboratory) we can tell whether it is redshifted or blueshifted and then we can calculate the velocity of the galaxy using the Doppler equation.
In the picture below left you can see information received from galaxy 1 and in the box below right you can see the wavelengths measured from the same spectra as from the galaxies in the laboratory. Ha is longer that the same line in the laboratory and this means the light has been redshiffted. Galaxy 1 is moving away from us.
Matthew, Aslam, Pameer and Wing Chung using the information given to calculate the recessional velocities of various galxies.
They also calculate the value of H0 for each row in their table (velocity ÷ distance) and took the average of these values. They wrote their average value at the bottom of their table.
Every measurement or observation has an uncertainty or error associated with it. The value for the Hubble constant calculated from the motion of one of the galaxies in the SDSS sample is 108.28 km s^‐1 Mpc^‐1.
There are a number of assumptions made in the derivation of this value and sources of error. The measured wavelength of Hα from the galaxy spectrum will have an uncertainty that is dependent on the resolution of the instrument used. For example if the measured wavelength is 7094 Å, the error, Δλ = 1 Å. There will also be an error in the laboratory wavelength which would be very small but here we will make it a bit bigger: rest wavelength of Hα = 6562.790 Å, error, Δλ0 = 0.001 Å. We use a value for the speed of light when calculating the velocity from the wavelength, c = 299 792 458 m s^‐1.
The students were given a list of data about galaxies and asked to plot a graph of distance from us (Mpc) against recessional velocity. As v = H0d then the gradient of the graph will be equal to the Hubble constant Ho.
The SI unit of H0 is s^−1 but it is most frequently quoted in (km/s)/Mpc
The inverse of the Hubble constant Ho has the units of time because the Hubble law is v = Hod where v is the velocity of recession, H is the Hubble constant, and d is the distance. Thus, from this equation, we have that 1/H = d/v. but d/v is distance divided by velocity, which is time (e.g., if I travel 180 km at 60 km/hour, the time required is t = d/v = 180/60 = 3 hours).
Thus, the Hubble time T is just the inverse of the Hubble Constant:
T = 1 / Ho providing the unit of H0 is s^-1.
The students compared their various values for the Hubble constant.
The values are diﬀerent because one was calculated using values of H0 from each of the galaxies and the other was taken from a best ﬁt line through the points. This method is more accurate as it uses the whole sample to get a truer estimate of H0 and not individual galaxies where the uncertainty is much larger.
The observational constraint related to galaxies that puts an absolute lower limit on the age of the Universe is that the Hubble telescope cannot see far back enough. The most distant galaxy observed must is younger than the age of the Universe i.e. the oldest galaxy seen so far (by the Hubble Space Telescope) is 13 billion light years away, this is 0.7 billion years less than the current value for the age of the Universe.
A larger value of the Hubble constant would give a younger age for the Universe. A steeper gradient of the velocity‐distance graph would mean galaxies have greater velocities at a certain distance, this could suggest a larger acceleration in the expansion of the Universe which may mean a lower density of matter or a greater repulsive force from something like dark energy.
The Hubble telescope was launched in the 1970s. Its orbit outside the distortion of Earth’s atmosphere allows it to take extremely high-resolution images with almost no background light. Hubble’s Deep Field has recorded some of the most detailed visible-light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.
Latest value for H0
The most accurate value for the Hubble constant has been determined using a diﬀerent technique to Hubble. A satellite called the Wilkinson Microwave Anisotropy Probe (WMAP) has been measuring temperature ﬂuctuations as small as 0.0002 K in the left‐over radiation from the Big Bang (called the cosmic microwave background, CMB) since 2001. Ripples in the CMB indicate the initial conditions for the formation of galaxies and reveal the shape and fate of the Universe.
The European successor to WMAP, a satellite called Planck is currently mapping the sky using radio receivers operating at very low temperatures. They will reveal anisotropies (temperature diﬀerences) in the CMB to a resolution of 1 microkelvin and will determine a more precise value for H0.
The students with Tom Kerss, who ran the session
Galaxies and Cosmology
The students with Catherine who gave the talk
Edwin Hubble made the two most important discoveries in cosmology. First, he proved that many nebulae are other “island universes” or galaxies, beyond the boundaries of the Milky Way. Then, working alongside colleagues at the Mount Wilson Observatory, he discovered that these galaxies are moving apart from one another – in effect that the universe is expanding.
By the start of the 1930’s, Hubble and his team had enough information to show that there was a straightforward relationship between the distance between us and a galaxy and the amount of redshift.
An implication that galaxies are moving away from us was that the Universe must be expanding and if that is so then there must have been a time in the past when it was smaller. If you can go back in time as far as possible you will come to the point when the Universe was concentrated at a single point. Using Hubble’s Law and the latest calibration of the redshift–distance relation, we can calculate that this corresponds to a time about 14 billion years ago.
The big bang is considered to be the start of our Universe and its age is based on measurements of expansion using Type 1a supernovae, measurements of temperature fluctuations in the cosmic microwave background and measurements of the correlation function of galaxies. The agreement of these three independent measurements strongly supports the model that describes in detail the contents of the universe.
The cosmic microwave background (CMB) is the thermal radiation assumed to be left over from the “Big Bang” of cosmology. It is the oldest light in the Universe, dating to the point where charged electrons and protons first became bound to form electrically neutral atoms. These atoms could no longer absorb thermal radiation and the Universe became transparent,
All-sky map of the CMB, created from 9 years of WMAP data
The cosmic microwave background radiation and the cosmological redshift-distance relation are together regarded as the best available evidence for the Big Bang theory. Measurements of the CMB have made the inflationary Big Bang theory the Standard Model of Cosmology.
Planck was a space observatory operated by the European Space Agency (ESA), and designed to observe anisotropies of the cosmic microwave background (CMB) at microwave and infra-red frequencies, with high sensitivity and small angular resolution. Planck had a higher resolution and sensitivity than WMAP, allowing it to probe the power spectrum of the CMB to much smaller scales (×3). It also observed in 9 frequency bands rather than WMAP’s 5, with the goal of improving the astrophysical foreground models.
Comparison of CMB results from COBE, WMAP and Planck.
Most of the cosmic microwave background is smooth but there are some ripples showing the regions where gravity intensifies to form stars. Our Sun formed nine billion years after the big bang.
Supermassive black holes, galaxies and galaxy clusters have all evolved together over the history of Universe.
The observable Universe today contains galaxies and other matter that can be observed from Earth today, but this only makes up about 4% of the Universe. It has a radius of about 46 billion light. But its actual size could be infinite.
The Universe is believed to be mostly composed of dark energy and dark matter, both of which are poorly understood at present. Less than 5% of the Universe is ordinary matter, a relatively small contribution.
In cosmology and physics, cold dark matter (CDM) is a hypothetical form of matter (a kind of dark matter) whose particles move slowly compared to the speed of light (the cold in CDM) and interact very weakly with electromagnetic radiation (the dark in CDM). It is believed that approximately 80% of matter in the Universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets and living organisms. As of 2006, most cosmologists favour the cold dark matter theory as a description of how the Universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today — the large-scale structure of the Universe. The theory sees the role that dwarf galaxies played as crucial, as they are thought to be natural building blocks that form larger structures, created by small-scale density fluctuations in the early Universe. Dark matter gravitates as ordinary matter, and thus works to slow the expansion of the Universe.
In physical cosmology and astronomy, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is expanding at an accelerating rate. According to the Planck mission team, and based on the standard model of cosmology, on a mass–energy equivalence basis the universe contains 26.8% dark matter and 68.3% dark energy (for a total of 95.1%) and 4.9% ordinary matter.
The fate of the universe is determined by the density of the universe. The quantity of evidence to date, based on measurements of the rate of expansion and the mass density and the fact that that the Universe appears to consist mainly of dark energy favours a universe that will continue to expand indefinitely, resulting in the “big freeze” scenario below. However, observations are not conclusive, and alternative models are still possible
A typical galaxy contains 100 billions of Suns and has a size of about 100000 light years.
A galaxy cluster contains about 10 trillions of Suns and has a size of about 10 million light years.
The size of a black hole is proportional to galaxy size. A black hole may stop a galaxy growing. There are three types of black holes.
Supermassive black holes (see http://en.wikipedia.org/wiki/Supermassive_black_hole)
A supermassive black hole is the largest type of black hole in a galaxy. Its mass is equivalent to billions of our Suns and its size is about 1 light year. It is too small to see but it is extremely powerful. Until recently they were thought to be very rare but we now know that they hide inside every galaxy. They are a phase not an oddity.
About 1% of galaxies contain an Active Galactic Nucleus (AGN) which consists of a supermassive black hole feeding on the gas from the galaxy and emitting huge quantities of radiation. It is thought that all galaxies go through an AGN phase.
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 and 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.
Supermassive black hole = 2.74 light hours across and galaxy is 325000 light years across. This is like saying the SMBH is a grain of sand and the galaxy is the Earth.
All three types of black hole (stellar, supermassive and miniature) evolved together. Jets from black holes may be due to spinning.
Well tested theories tell us how the universe formed and how it has evolved. Gravity is involved.
The first galaxies were believed to have formed 200 million years after the big bang. Our Sun formed 9 million years ago.
Computer simulations can be used to model the creation of the universe but the properties of simulated galaxies and clusters don’t always match the observed universe.
The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Formax and allows us to see far back in time when the galaxies first appeared after the big bang.
In this session students learnt how astronomers determine the properties of distant stars by examining spectra and applying their knowledge of the electromagnetic spectrum, the reflection, absorption and emission of light, and the Doppler Effect.
The above pictures show that our bodies give off radiation. Infra-red radiation is able to pass through the black plastic bin bag. The aim of this activity was to show that the radiation given off by stars can give information about their temperature.
A spectrum is a ‘fingerprint’ of an object made of light. The spectrum of visible light is composed of the colours of the rainbow.
The different parts of the electromagnetic spectrum are: gamma ray, X‐ray, ultraviolet, visible, infrared, microwave, radio.
These different types of light differ because they have different wavelengths or frequencies which give them different energies e.g. gamma rays have a short wavelength, a high frequency and a high energy, radio waves have a much longer wavelength, a low frequency and consequently a low energy.
X‐rays and gamma rays have high energy radiation which can damage our DNA and our cells; we would become very ill and eventually die from exposure to this radiation. Even though the Sun emits (high energy) X‐rays and gamma rays life evolved on Earth Life and survived because we have an atmosphere that protects us from this radiation.
We have telescopes for the whole of the electromagnetic spectrum and not just for optical light because we can’t see everything with just optical light, often this light is blocked by dust or there are regions in space that do not emit optical light but appear bright in other wavelengths.
Satellites such as Yohkoh, SOHO (Solar and Heliospheric Observatory) and terrestrial telescopes such as the McMath‐Pierce telescope on Kitt Peak in Arizona have imaged the Sun in X‐ray, ultraviolet and infrared light. X‐ray light has the shortest wavelength, infrared is the longest.
The dark patches in the X‐ray and UV images are bright in the IR (these are called coronal holes, regions in the Sun’s outer atmosphere ‐ the corona where magnetic field lines burst through). The dark features in the IR image are bright in shorter wavelengths. There is absorption of IR light by the gas in these regions.
Examples of solar filters are the hydrogen alpha (Hα) which transmits a wavelength of 656.3 nm, the sodium D, wavelength = 589 nm and calcium K filters, wavelength = 393 nm. Through the Hα filter the Sun is red, Na D is yellow and Ca K is blue.
In the X‐ray and visible wavelengths the Milky Way appears dark with brighter regions above and below, particularly in the X‐ray. In contrast the galaxy is very bright in gamma ray and in the opposite region of the electromagnetic spectrum at the longer wavelengths of infrared to radio – here vast diffuse bright regions can be seen around the galaxy.
There is a lot of dust in the Milky Way which blocks visible light from reaching us, however high energy (short wavelength) and low energy (long wavelength) light can penetrate this dust thus our galaxy appears brighter in these wavelengths. Also there is a lot of gas in the Milky Way at a large range of temperatures (and different energies) thus emerging different wavelengths of light.
It important for astronomers to look at objects in space in all wavelengths so we don’t miss anything! Features we can’t see in visible light we can see in other wavelengths, this way we get a complete picture of the object we are studying.
The Sun appears yellow in colour in visible light. We cannot see the Sun in wavelengths outside of the visible region of the electromagnetic spectrum with our eyes however we can build detectors and telescopes that respond to light of other wavelengths. The Sun looks very different in other wavelengths (below). These have been artificially coloured so that we can see them. Satellites such as Yohkoh, SOHO (Solar and Heliospheric Observatory) and terrestrial telescopes such as the McMath‐Pierce telescope on Kitt Peak in Arizona have imaged the Sun in X‐ray, ultraviolet and infrared light.
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.
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.
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.
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 X-ray 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.
Stars and star light
Stars are objects that produce enough energy (via nuclear fusion in their core) to maintain their size against gravitational collapse.
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.
A star much bigger than our Sun produced heavy elements in its core before becoming a supernova which is how we have elements heavier than iron.
The ejected gas from supernovae clumped together over time to form a Solar System like ours – all of the elements we have on Earth come from stars.
The atmosphere 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 which is why we need telescopes in space.
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 spectra and the star will be redder.
Hydrogen (74%), helium (25%), lithium, calcium, magnesium, sodium, iron (these make up 1% of the Sun).
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.
Most stars are currently classified under the Morgan–Keenan (MKK) system using the letters O, B, A, F, G, K, M, L, T and Y, a sequence from the hottest (O type) to the coolest (Y type). The types R and N are carbon-based stars, and the type S is zirconium-monoxide-based stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler).
Wien’s Law gives a relationship between the peak wavelength of a star’s spectrum and its temperature (in kelvin).