Twenty-five years ago the Hubble space telescope took its first image of the heavens. While the first image might not have seemed impressive at first glance, it opened a view of the universe previously unattainable.

First Hubble image (right) compared to a ground-based image (left).

The Hubble telescope had several advantages over ground-based telescopes of the time. To begin with, it didn’t suffer from the atmospheric distortions that cause stars to twinkle. Because of such distortions, ground-based telescopes had a limiting resolution of about a second of arc. The Hubble could gather light to about 0.05 arcseconds. Since Hubble’s launch, ground based telescopes have gained greater precision through interferometry and adaptive optics, but these were years away when Hubble’s mission began. Hubble could also observe infrared and ultraviolet wavelengths, much of which are absorbed by Earth’s atmosphere.

The greater range and precision of Hubble has given us a much deeper understanding of the universe. It allowed us to measure Cepheid variable stars with greater precision, which gave us a more accurate cosmic distance ladder. It found protoplanetary disks in the Orion nebula, confirming the basic model of planetary formation. It gave us the Ultra Deep Field, which found that there are thousands of galaxies in a patch of sky no bigger than a grain of sand. It also gave us thousands of beautiful images, from the Pillars of Creation to the starry ocean of the Andromeda galaxy.

Hubble’s deployment from Discovery. Credit: NASA/IMAX

But perhaps its most revolutionary discovery came from observations of distant supernovae. By measuring the brightness of these supernovae, we could determine their distance. What we found was that the universe is not only expanding, but that the rate of expansion is increasing. This not only gave us an accurate measure of the age of the universe, it also led to the discovery of dark energy, which we are still trying to understand.

The Hubble telescope was named in honor of Edwin Hubble, who played a central role in demonstrating that the universe extended far beyond our Milky Way galaxy. It is perhaps fitting that the telescope bearing his name has done so much to determine the true age and scale of the cosmos.

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When you look up in the night sky, there are areas of the sky that appear dark.  That’s because there is nothing in that region bright enough for us to see with the naked eye.  If you looked upon this region with a telescope, you would find dim stars and galaxies, but you would still see areas that appeared dark to you.  How far could you take this? If you kept looking at smaller and smaller dark regions with ever more powerful telescopes, what would you see?

Starting in 2003, we’ve put this idea to the test when the Hubble telescope was aimed at one of the darkest patches of sky we could find, in the Fornax constellation.  It is an area so small, it is the apparent size of a grain of sand held at arm’s length.  Off and on over the next 9 years, the Hubble observed this region, gathering ever more light to see details in the image. Hubble gathered light at infrared and visible wavelengths, and most recently gathered ultraviolet wavelengths as well. After gathering light for about 1200 hours total, what you get is this:

2014 Hubble Ultra Deep Field. Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI),
R. Windhorst (Arizona State University), and Z. Levay (STScI)

There are about 10,000 galaxies in this image.  10,000 galaxies in the area of a sand grain.  This is what lies within the dark of night.

From this image we’ve estimated that there are 100 billion galaxies in the visible universe. That’s more than 10 galaxies for every man, woman and child on Earth. Those galaxies might have an average of about 100 billion stars. Around most of those stars might be tens of planets. It is a vast sea in the dark of night.

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One of the advantages of modern astronomy is that most observational data gets stored in a raw form.  This is particularly true for the major space telescopes.  Most of that raw data is also stored publicly, either a certain time period or even as the data is gathered.  This means that long after an observation is made, people can go through the data to analyze it in new ways.  As a case in point, a team recently gathered old data from the Hubble Space Telescope, and processed it using new methods.  From these they discovered debris disks around five stars.

The data on these stars was originally gathered between 1999 and 2006.  Earlier, infrared observations of these stars by the IRAS and Spitzer telescopes showed them to have an unusual signature that could indicate a debris disk around the stars.  So detailed observations of the stars were made with Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS), but there was no clear debris disks to be seen.  Part of the challenge with this kind of observation is that you want to observe infrared light scattering off the debris disk. That disk is orbiting a star which is much brighter in infrared, so you need to block the star while still taking sensitive measurements.  The resulting data can be pretty noisy, which makes it hard to distinguish data from noise.

But over the past decade computers have gotten more powerful and the methods of extracting signal from noise have gotten better.  So this team went back to the original data and re-analyzed it.  Their results were published in the Astrophysical Journal Letters this month. The results clearly show scattered light debris disks around these stars.  This actually increased the number of known scattered light disks by almost a factor of 3. The team was also able to determine the size and orientations of these disks, which is pretty impressive.

Asymmetrical debris disk around a Sun-like star.
Credit: NASA / ESA / R. Soummer, Ann Feild, STScI.

One of the stars, with the memorable name HD 141943, is very similar to the early Sun.  It’s debris disk is asymmetrical, which suggests that there are proto-planets gravitationally influencing it.  One of the things the team plans for future research is to further process the data to resolve these proto-planets.  If they are successful it could lead to new insights on how exoplanetary systems form.

What is particularly cool about this research is that it used publicly available data.  You can actually go to the Mikulski Archive for Space Telescopes and access this kind of data yourselves, at no cost to you.  If you use the data in a research project, all you need to provide is an acknowledgement in your paper.  There is a wealth of data there, and all it needs is a second look.

Paper: Remi Soummer, et al. FIVE DEBRIS DISKS NEWLY REVEALED IN SCATTERED LIGHT FROM THE HUBBLE SPACE TELESCOPE NICMOS ARCHIVE. The Astrophysical Journal Letters, 786:L23 (2014)

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Edwin Hubble is perhaps most famous for discovering a relationship between the distance of a galaxy and the speed at which a galaxy moves away from us. This relation is now known as Hubble’s Law, and is evidence for the expansion of the universe. But Hubble’s primary interest was in galactic nebulae (what we now just call galaxies).

Hubble was a consummate astronomer, and made some of the best galactic observations of his day. Over time he noticed that while galaxies could have all sorts of different shapes, they could be grouped into three large classes: ellipticals, spirals, and irregular. Hubble further organized galaxies into sub-categories, and in 1926 proposed a classification scheme known as the Hubble tuning fork.

Hubble’s tuning fork diagram for galaxies.
Credit: Edwin Hubble

You can see this classification in the image here. You can see that starting with an almost spherical elliptical galaxy at the left, they get gradually flatter and disk-shaped, reaching s0, which is an almost uniform disk. These are also known as lenticular galaxies. Then the chart splits into spiral types above and barred spiral below.

If this looks like a description of the evolution of galaxies, that’s understandable. Hubble recognized the transition from elliptical to spiral as a possible path of galactic evolution, but he cautioned readers not to make assumptions. We now know galaxies do not evolve from elliptical to spiral. The most distant observed galaxies (and therefore the earliest galaxies) are primarily spirals and irregulars. It is now thought that elliptical galaxies are the result of mergers between galaxies.

Hubble’s tuning fork classification is still used today. As we have discovered more galaxies, and our observations of these galaxies have become more precise, we’ve found the categories are not a hard and fast rule. Determining exactly which into category a specific galaxy falls can be a bit of a judgement call. Still, knowing a galaxy’s classification is a good way to get a handle on its broad characteristics.

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One of my favorite objects in the night sky is the Orion nebula.  It’s easy to find, just below the belt stars in the constellation Orion, and it is the closest stellar nursery.  It’s also a bit bigger than the Moon.  Actually that’s a bit misleading.  It is actually about 24 light years across, but because it is also about 1300 light years away, its apparent diameter is about a degree, which is about twice the apparent diameter of the Moon.

Credit: The Wondrous Sky.

In astronomy, apparent size is important because it’s one of the factors that determines our ability to observe them in detail.  With any telescope, there is a limit to how close two features can be before they blur together.  This is known as its resolving power.  For a simple telescope, this is a function of the diameter of its main mirror or lens (all other things being equal).  A 4″ telescope, for example, has a resolving power of about 2 arcseconds.  An arcsecond is 1/3600 of a degree, so using such a telescope to look at the Moon would be kind of like looking at an image of the Moon about 900 pixels wide.

Of course there are other factors as well.  The resolution power of telescopes also depends on the wavelength at which you are observing, then there is the issue of how much light pollution you have in your area, whether the air above you is calm or turbulent, and others.  Usually a backyard telescope can’t resolve things to their theoretical limit.

Moon compared to Hubble Extreme Deep Field. Credit: NASA/Hubble

One way to overcome many issues is to put your telescope in space.  The Hubble space telescope is above the atmosphere, so it can resolve things at close to its resolution power, which is about 100 milliarcseconds.  This is one of the reasons we put telescopes in space.

One  way to overcome the resolution limit is to use multiple telescopes.  This process is known as  interferometry.  The light gathered from multiple telescopes can be combined to create a “virtual” telescope.  The greater the virtual size, the more resolution you can have.  This particularly useful with radio telescopes.  The largest of these is the Very Long Baseline Array, which has an effective diameter of over 8,000 kilometers.

When things are billions of light years away, even large objects have a tiny apparent diameter.  This is why we’re always striving for greater resolution in our telescopes.

It’s all a matter of scale.

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If you look in the constellation of Andromeda on a dark night, you can see a small faint smudge that looks a bit like a smudge of chalk against the starry blackness. The image below gives a good idea of its appearance to the naked eye.  This object is known as M-31, named after Charles Messier who first cataloged these fuzzy objects in the 1770s.

Credit: P. Clay Sherrod

In the 1700s, it was clear that the Messier objects such as M-31 are not stars. They are also not comets, as they they don’t move through the sky.  Messier actually cataloged these objects so he wouldn’t confuse them with comets, which also look like fuzzy patches in the sky.

At the time M-31, was known as a nebula.  The known universe then was thought to be similar in structure to what we now call the Milky Way galaxy, so Messier objects were thought to be regions of gas and dust.  Some Messier objects are just that, but in the mid-1800s it was noticed that M-31 had a light spectrum that looked more like a collection of stars than a nebula.  By the late 1800s, photographic observations confirmed that M-31 was a cluster of stars, not gas and dust.

In 1925 Edwin Hubble took careful measurements of variable stars in M-31 known as Cepheid variables.  These stars vary in brightness at a rate proportional to their absolute magnitude.  By measuring the changes in their brightness, Hubble determined their distance.  What he found was that M-31 was about 2 million light years away, and not part of our own galaxy.  Hubble demonstrated that the universe contained island galaxies in a vast empty space, and our galaxy was just one among many.

M-31 is now more commonly known as the Andromeda galaxy.  At 2.5 million light years away, it is the most distant object that can be easily observed with the naked eye.  The Triangulum galaxy (M-33) is a bit more distant, but fainter.  So if you get a chance to glimpse the Andromeda galaxy some night, you’ll know you’re looking at island of stars in the cosmos.

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Suppose you picked up a grain of sand and held it at arm’s length. If you held it up in the night sky, it would block a tiny fraction of the visible heavens.  Now suppose instead of a sand grain it were a tiny window, through which you could see even the faintest light.  Finally, suppose you were to take your tiny window and point it at the darkest patch of night you could find.  What would you see?

Of course we have such a “window,” called the Hubble telescope, and we’ve done just what I’ve described several times.  This time we aimed it at one of the darkest patches of sky we could find, in the Fornax constellation.  After gathering light for a total of almost 23 days, what we got was the image below, just released today.  It is known as the Extreme Deep Field.

Take a moment to let the profound nature of this image sink in.  This image is what we got when we pointed the Hubble telescope at what looked like empty space.  Each smudge of light in this image is a young galaxy, from about 500 million years after the big bang.  Thousands of  galaxies in a patch of sky the size of a grain of sand.

Of course there isn’t anything particularly special about the direction we looked other than the fact that there wasn’t anything in the way.  If we looked in any other direction we would see basically the same thing.  Imagine the sky covered with grains of sand, and in each sand grain thousands of galaxies.  It’s estimated that there are 100 billion galaxies in the visible universe.  That’s more than 10 galaxies for every man, woman and child on Earth.  Those galaxies might have an average of about 100 billion stars.  Around most of those stars might be tens of planets. Countless cosmic grains of sand.

More in heaven and earth, indeed.

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A while back I wrote about the relationship between the distance of a galaxy from us and the speed at which it moves away from us.  This relationship is now known as Hubble’s law, after Edwin Hubble, who first plotted the speed of a couple dozen galaxies versus their distance in 1929.  What he found was a linear relationship between speed and distance.  In other words, it seemed the speed of a galaxy divided by its distance was a constant, now known as the Hubble constant.  It was the first solid evidence of an expanding universe.

Since Hubble’s day we’ve been able to measure the speeds and distances of more than four thousand galaxies, so we’re able to get a much better measurement of the Hubble constant.  All this extra data has made things very interesting.

For one thing, as we measure more distant galaxies we have to change Hubble’s law a bit.  Hubble’s original relation was between speed and distance, but at really large distances galaxies are moving away from us at a large fraction of the speed of light.  This means we have to take special relativity into account at large distances.  For this reason the Hubble relation is now expressed not in terms of galactic speed, but in terms of a measure of redshift known as z.  The nice thing about z is that it allows special relativity to be basically factored out of observations.  If Hubble’s model holds, then a plot of z versus distance should be a straight line.

A wonderful aspect of observational astronomy is that when you look at more and more distant objects, you are also looking further back into time.  If a galaxy is a billion light years away from us, the light we observe left that galaxy a billion years ago.  This means galactic distance is a also a measure of the past.  As a result, the Hubble constant is not just a measure z versus distance, but also a measure of z versus time.  In other words it tells us the speed at which the universe is expanding.

Redshift vs distance.

In the past decade, however, we’ve found that Hubble’s constant doesn’t quite hold, as you can see in the figure above.  The linear relation is plotted as a black line, but the best fit to the data is the red line.  The red line isn’t straight, but instead curves slightly upward. This means at really large distances the redshift is greater than we would expect.  The expansion speed of the universe is not constant, but is getting larger.  In other words, the universe is accelerating.

Of course this means something is likely causing this acceleration. That something is known as dark energy.

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One of the more interesting astrophysical discoveries of the 20th century is the fact that the universe is expanding. The result was so unexpected that even Einstein discarded its prediction within general relativity. Einstein went so far as to introduce an extra constant in his equations specifically to prevent an expanding universe model. He would later call it his greatest blunder.

But how do we know the universe is actually expanding? For this we need to use the handy-dandy Doppler effect. You might remember that the observed color of light can be effected by the relative motion of its source. If a light source is moving toward us, the light we see is more bluish than we would expect (blue shifted). If a light source is moving away from us, the light is more reddish (red shifted). The faster the source is moving, the greater the shift.

We have measured this color shift for lots of stars, galaxies and clusters. We’ve also determined their distances (exactly how will be a post for another day). If we plot a graph of the distance of galaxies and clusters versus their redshift we find something very interesting. I’ve plotted such a graph below, and you can see there is almost a linear relationship between distance and redshift.

Distance vs speed for galaxies.

This means galaxies are not simply moving at random, as you would expect in a stable, uniform universe. Instead, the more distant the galaxy the faster it is moving away from us. This relation between distance and speed is the same in all directions, which means the universe seems to be expanding in all directions.

Since this relationship is linear, you can fit this data to a line. The slope of the line is known as the Hubble constant, named after Edwin Hubble, who was one of the first to observe this relationship. When I did a simple linear fit to the data (the dashed line), I got a Hubble constant of 68.79 km/s per megaparsec. This is in the range of the accepted value.

Of course if the universe is expanding, then it must have been smaller in the past. If we assume the universe expands at a constant rate, then we can trace its size back in time to a point where the universe would have zero volume. In other words, the universe has a finite age, and it began very small, very dense (and therefore very hot). We call that starting point the big bang. If you do the math, the age of the universe is simply the inverse of the Hubble constant. Given our value, this puts the age of the universe at about 14.5 billion years. More accurate calculations put the age at 13.75 billion years.

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