Posts Tagged ‘Edwin Hubble’

A few weeks ago it was announced that a team had discovered what seems to be the most distant galaxy yet discovered. You can read the BBC story about it here, or if you like you can read the Nature science paper here to get as much detail as you could wish for. The galaxy, which has the catchy name z8_GND_5296, was discovered using the Hubble Space Telescope, with its distance being determined using the Keck 10m telescope on the summit of Mauna Kea.

In fact, what astronomers measure is not the distance of a distant galaxy, but its redshift, which astronomers denote with the letter z. Redshift is the movement of the spectral lines of a galaxy to longer wavelengths due to the expansion of the Universe, the expansion discovered by Edwin Hubble in 1929. The redshift of this newly discovered galaxy has been found by Keck to be z=7.51, beating the previous record of z=7.21. But how do astronomers translate this into a distance?


The cosmological definition of redshift

It turns out that measuring distances in astronomy is one of the most difficult things to do for several reasons. Not only are there very few direct ways to measure the distance to an object, after all we can hardly lay down a measuring tape between us and the stars and galaxies! But, to make it even worse, there also are various definitions of distance! In a future blog I will talk about the most direct ways we have to measure distance, but how we translate from these measurements to a distance also depend on the geometry of the Universe, which Einstein showed in his General Theory of Relativity is determined by the effects of gravity.

The geometry of the Universe is determined by its average density \Omega, and how this relates to something called the “critical density” \Omega_{0}, which is the dividing line between whether the Universe will carry on expanding forever, or stop expanding and start to collapse. If average density \Omega > \Omega_{0} the Universe will stop expanding and collapse. If \Omega < \Omega_{0} the Universe will carry on expanding forever, and if the average density \Omega = \Omega_{0} the Universe is on the dividing line between the two, and is said to have a flat geometry. Without going into the details here, most cosmologists believe that we live in a Universe where \Omega = \Omega_{0}, that is a flat Universe.

The preferred method for measuring large distances “directly” is to use something called a Type Ia Supernova, I will blog about this method again in a future blog. But, we can only see Type Ia supernovae out to distances corresponding to a redshift of about z=1. The galaxy in this story is much further away than this, z=7.51. So, to calculate its distance we have to use a model for the expansion of the Universe, and something called Hubble’s law.

The measured redshift of a galaxy (or any object) is just given by

z = \frac{ \lambda - \lambda_{0} }{ \lambda_{0} } \text{ (Eq. 1) }

where \lambda is the observed wavelength and \lambda_{0} would be the wavelength of a spectral line (usually for a galaxy it is a line called the Lyman-alpha line) in the laboratory.

As long as the redshift is much less than 1, we can then write that

z=\frac{ v }{ c } \text{ (Eq. 2) }

where v is the recession velocity of the galaxy and c is the speed of light. In the case of z not being less than 1, we need to modify this equation to the relativistic version, so we write

1 + z = \sqrt{ \frac{ 1+ v/c }{ 1 - v/c } } \text{ (Eq. 3) }

In our case, z=7.51, so we need to use this relativistic formula, and when we do we get that the recession velocity of the galaxy is 97\% \text{ of c }, the speed of light.

Re-arranging equation 1 we can write 1 + z = \frac{ \lambda }{ \lambda_{0} }. In principle, the distance and redshift are just related via the Hubble law

v = H_{0} d \text{ (Eq. 4) },

where v is the recession velocity of the galaxy, H_{0} is the Hubble constant, and d is the distance of the galaxy.

Things get a lot more complicated, however, when we take into account General Relativity, and its effects on the curvature of space, and even the definition of distance in an expanding Universe. I will return to this in a future blog, but here I will just quote the answer one gets if one inputs a redshift of z=7.51 into a “distance calculator” where we specify the value of Hubble’s constant to be H_{0} = 72 \text{ km/s/Mpc } and we have a flat Universe (\Omega=1) with a value of \Omega_{M}=0.25 (the relative density of the Universe in the form of matter) and \Omega_{vac} = 0.75 (the relative density of the Universe in the form of dark energy).


Putting these values in gives a co-moving radial distance to the galaxy of 9103 Mpc \text{ or } 29.7 \text{ billion light years}. (I will define what “co-moving radial distance” is in a future blog, but it is the distance quoted in this story, and is the measurement of distance which is closest to what we think of as “distance”).

The redshift also gives a time when the galaxy was formed, with z=0 being the present. We find that it was formed some 13.1 billion years ago, when the Universe was only about 700,000 years old.

A galaxy 30 billion light years away??

Going back to the “co-moving radial distance”, I said it is about 30 billion light years. A light year is, of course, the distance light travels in one year. So how can a galaxy be 30 billion light years away, implying the light has taken 30 billion years to reach us, if the Universe is only 13.7 billion years old?? This sounds like a contradiction. The solution to this apparent contradiction is that the Universe has expanded since the light left the galaxy. This is what causes the redshift. In fact, the size of the Universe now compared to the size of the Universe when the light left the galaxy is simply given by

1 + z = \frac{ a_{now} }{ a_{then} }

where a is known as the scale factor of the Universe, or its relative size. For z=7.51 we have a_{now} = (1 + 7.51)\times a_{then} = 8.51 a_{then}, so the Universe is 8.51 times bigger now than when light left the galaxy (this is what causes the redshift, it is the expansion of space, not that the galaxy is moving through space with a speed of 97% of the speed of light). It is the fact that the Universe is over 8 times bigger now than when the light left the galaxy which allows its distance measured in light years to be more than a distance of 13.7 billion light years that one would naively think was the maximum possible! So, there is no contradiction when one thinks about things correctly.

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Last Thursday (24th of October 2013) I gave a talk to Swansea Astronomical Society. This is the third year in a row that I have spoken in the autumn to this wonderfully active society on a historical theme. Two years ago I spoke about the early history of Yerkes Observatory (I blogged about that talk here), and last year I spoke about George Ellery Hale (my blog on that talk is here).

This year I continued the Hale theme, speaking about the history of Mount Wilson Observatory, which Hale established in 1904 after resigning as Director of Yerkes Observatory. Mount Wilson Observatory is most famous of course for its 100-inch telescope, the telescope used by Hubble (and Humason) to discover that the Universe is expanding. The Observatory is located just outside Los Angeles, and despite the light pollution of LA, it is still a very active observatory. This is mainly due its exceptionally stable air, giving it image quality better than pretty much any other observatory in the continental USA.

My connection with Mount Wilson Observatory is not as strong as my connection with Yerkes, but I was lucky enough to be awarded a Mount Wilson Fellowship in late 1999 and so went to use the famous 100-inch on four separate observing runs in 1999/2000. I was using an adaptive optics system, the plan was to study in unprecedented detail the structure of the scattering of visible light from dust grains in reflection nebulae. Unfortunately we were not able to use the AO system to do this work, as the central stars illuminating the reflection nebulae were too far from the dust regions we wanted to study for the AO system to work. In addition, our primary target, NGC 7023, is located at too high a declination for the 100-inch with its yolk mount to be able to reach. I thus undertook an alternative observing programme of observing close binary star systems to determine their orbital properties, systems which were too close to be resolved with conventional telescopes not using an AO system.

During all of these four observing runs I do not remember seeing the stars twinkle when it was clear (which it was most nights), which is testimony to the incredible seeing the Observatory enjoys. Even way down towards the horizon, the stars remained rock steady to the naked eye. It is because of this exceptional seeing that Mount Wilson was the testing ground for Adaptive Optics systems, and is now the testing ground for optical interferometry, with projects like the CHARA project run by Georgia State University (see this link for more information).

Here are the slides from my talk. I hope you enjoy them, and of course if you have any questions please feel free to ask in the comments section.

Here is a video of my talk. Apologies for the quality.

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In this blog I discussed the Messier catalogue, including its historical background. As I mentioned in that blog, many of the 110 objects in the Messier catalogue are amongst the most beautiful and interesting objects in the sky. In a series of blogs I am going to discuss some of my favourite objects in the Messier catalogue in more detail, including how to find them.

First off, Messier 31.

Messier 31 – The Andromeda galaxy

I have already talked about this galaxy a few times before. In this blog I explained how Edwin Hubble showed in 1923 that M31 lies outside of our Milky Way galaxy. It was the first object ever shown to lie outside the Milky Way, and of course led to the realisation that many similar objects also lay beyond our Galaxy. This discovery by Hubble in 1923 totally changed our perception of our Universe and our place in it. It provided conclusive proof that our Milky Way galaxy was not the entire Universe, but just one galaxy in many. In my blog introducing the Messier catalogue it was one of the four objects I illustrated.

Messier 31 is an example of a spiral galaxy. However, in visible light its spiral arms are not very prominent. Along with our Milky Way, Messier 31 and our home Galaxy form the two largest galaxies in our Local Group. There are other galaxies in the local group, but they are all so-called dwarf galaxies and are either satellite galaxies of our Milky Way or of M31.

Messier 31 - The Andromeda galaxy. This is the most distant object that can be seen with the naked eye.

Messier 31 – The Andromeda galaxy.


Our current best measurements suggest that M31 is about 2 million light years away. That is, the light has taken 2 million years from when it left M31 to reach us. You are seeing the object as it was 2 million years ago! This makes it the most distant object visible to the naked eye. However, to see it with the naked eye you have to be (a) in a dark place away from light pollution and (b) know where to look. Here is a finding chart to find M31.

How to find Messier 31


Messier 31 is best seen in the late Summer and Autumn sky. The easiest way to find M31 is to first find the Square of Pegasus, an easily located asterism which forms part of the constellation Pegasus. The bright bluish-white star in the North-Eastern corner (top left as seen from the Northern Hemisphere) is called Alpheratz. If one moves to the left (East) from this star the Andromeda constellation forms a thin v-shape. The lower branch of the “v” has a bright red star called Mirach (the second star along from Alpheratz). From Mirach go up to pi-Andromodae (quite a faint star), and in a straight line an equal distance from Mirach to pi-Andromodae is where you should find the Andromeda galaxy.

However, even if you are in a dark place it is extremely difficult to see, and with your naked eye you will probably only be able to see it using your peripheral vision, the part of the eye which is more sensitive to low light levels. If you are having difficulty finding it with your eye, it is quite a good idea to find it with a low powered magnification in a telescope, and then use this to help you know what you are looking for with just your un-aided eye.

What you will see with your eye, whether un-aided or through a telescope, will probably not look much like the photograph above. Rather, you will see a fuzzy blob. Remember, the word “nebula” comes from the Latin word for cloud, and Messier 31 looks like a fuzzy cloud. Until astronomers gained a better understanding of objects in the Messier catalogue, a galaxy like Messier 31 was referred to as a “nebula” just as Messier 42, the Orion nebula, had the same term attached to it, even though we now know they are very different kinds of objects.

Messier 31 in ultra-violet light and infrared light

Although the spiral arms are not very prominent when we take an image of M31 in visible light, its appearance is quite different when we observe it in e.g. ultra-violet light and infrared light. The uv image below was taken by a mission called Galex, and shows a prominent ring of stars emitting in the uv part of the spectrum. The nucleus of the galaxy is also seen to be emitting quite strongly in uv light. It is only the hotter, younger stars which emit in the uv, cooler and older stars like our Sun are just not hot enough to emit much in ultra-violet light. The hot young stars are found in the spiral arms of spiral galaxies, in fact it is the presence of these hot young stars which give such galaxies their spiral appearance.


A prominent ring is also visible in this infrared image of M31 shown below. This image was taken with the Herschel Space Observatory, and was taken at a wavelength of 250 microns. What we are seeing in this image is not radiation from stars, but rather the emission from interstellar dust. At 250 microns we can see both warm dust heated by hot young stars and cooler dust that is heated by the more general stellar population. That is why you see you reasonable but not exact correspondence between the uv image and the infrared image.


Studying objects at different wavelengths has allowed to learn a lot more about them than if we just observe them with visible light. Nowadays, astronomers have the facilities to observe astrophysical objects from the X-ray to the radio part of the spectrum, and through doing this gain a much deeper understanding of the object than if it were just observed in visible light alone.

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In this blog I described the first results from the Planck satellite, which is studying the Cosmic Microwave Background in greater detail than we have ever done before. But, what exactly is the Cosmic Microwave Background? Where does it come from? How was it produced?

The origin of the elements

In 1929 Edwin Hubble published evidence that the speed with which galaxies were moving away from the Milky Way was directly related to their distance from us. Although Hubble himself never explicity stated it, this is clear evidence that the Universe is expanding. If the Universe is expanding, then of course one would expect it to have been smaller in the past.

In the 1940s the Russian-American physicist George Gamow started thinking about what the early Universe would have been like. He worked on two related theories, the first that the elements would have been created in the early Universe. The second related to the fact that a smaller, denser Universe would also have been hotter in the past.

In 1948, with his PhD student Ralph Alpher, the two published a paper titled “The Origin of Chemical Elements“. As a joke, Gamow decided to add the well-known physicist Hans Bethe’s name to the paper, so that it could be called “Alpher, Bethe, Gamow” (alpha,beta, gamma – geddit? 🙂 ).

George Gamow, who worked with his PhD student Ralph Alpher on the primordial nucleosynthesis theory.

George Gamow, who worked with his PhD student Ralph Alpher on the primordial nucleosynthesis theory.

Ralph Alpher, who was George Gamow's PhD student at the time of writing the paper.

Ralph Alpher, who was George Gamow’s PhD student at the time of writing the paper.

Hans Bethe, who played no part in writing the paper.

Hans Bethe, who played no part in writing the paper.

The famous "Alpha, Bethe, Gamow" paper from Physical Review 1948

The famous “Alpha, Bethe, Gamow” paper from Physical Review 1948

Although the Alpher, Bethe, Gamow paper was groundbreaking, it was wrong in some of its details. It suggested that all the elements were created in the hot, early Universe. We now think (know?) this is not the case. Only hydrogen and helium were created in the early Universe, the other elements have all been created inside stars, something Sir Fred Hoyle worked out with co-workers in the 1950s.

Alpher and Herman’s paper on the Cosmic Microwave Background

In a related paper, Alpher and Robert Herman, who was working as a post-doctoral research assistant for Gamow, calculated that the early Universe would have been a hot opaque plasma (ionised gas), and would thus have radiated like a black body. However, this radiation would not have been able to travel through the plasma as the photons would scatter of the free electrons.

The abstract of the paper by Alpher and Hermann, which predicts a cosmic microwave background at a temperature of 5K (5 degrees above absolute zero).

The abstract of the paper by Alpher and Herman, which predicts a cosmic microwave background at a temperature of 5K (5 degrees above absolute zero).

Gamow's article in Nature, which summarises the work on the origin of the elements and of the existence of a cosmic microwave background

Gamow’s article in Nature, which summarises the work on the origin of the elements and of the existence of a cosmic microwave background

But, as the Universe expanded and cooled the plasma would become a neutral gas, in that the electrons would combine with the nuclei to produce neutral atoms, allowing the photons to travel unimpeded. They calculated that these photons, which would be able to thence travel unimpeded, would now be at a characteristic black-body temperature of 5K due to the expansion of the Universe. This in the microwave part of the spectrum, hence the name Cosmic Microwave Background.

Our current understanding is pretty much what was derived in this 1948 paper, with a few refinements. Perversely, the moment the plasma became a neutral gas, which we believe to be when the Universe was about 350,000 years old, is referred to as “re-combination”, but as I tell my students, the electrons were combing with the nuclei for the first time. This is when the fog of the early Universe lifted and is the earliest radiation we can see.

In a separate blog on the history of the Cosmic Microwave Background (CMB) I will discuss how

  1. the CMB was accidentally discovered in 1964
  2. Gamow’s work was ignored, only to be worked out again in the early 1960s


You can read far more about the prediction of the CMB, and its accidental discovery, in my new book, “The Cosmic Microwave Background – How it changed our understanding of the Universe”.

My book "The Cosmic Microwave Background - how it changed our understanding of the Universe" is published by Springer and can be found by following this link.

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

The book can be found on the Springer website here, and on the Amazon website here.

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In 1923 Edwin Hubble showed that the Andromeda nebula was, in fact, a galaxy beyond our own Milky Way galaxy. What was the background to his being able to show this? It involved trips to the other side of the World to observe the rare Transit of Venus, a gifted woman astronomer whose boss got all the credit, and a pipe smoking egomaniac who had a fake Oxford English accent.

This is the story of how the Andromeda nebula became a galaxy.

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In November of last year (2011), I have a talk to Swansea Astronomical Society on the early history of Yerkes Observatory. I blogged about that talk here.

Last night (Thursday 8th of November) I gave a talk to the same society with the title “ George Ellery Hale : The greatest Astronomer of the 20th Century?“. The title is deliberately provocative. In the talk I attempted to show Hale‘s main achievements in his productive life. There were many, but this slide summarises the main ones :

A summary of Hale’s main achievements in his astronomical career

Here is a gallery of all the 32 slides in the talk.

My conclusion, in the last slide, is that maybe Hale wasn’t the greatest astronomer of the 20th Century, but probably the most important. Without Hale, Yerkes Observatory would never have existed, nor Mount Wilson Observatory.

Who do you think was the greatest astronomer of the 20th Century?

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