Starts With A Bang! » Hubble Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Happy Halloween from Hubble! Fri, 31 Oct 2008 14:27:29 +0000 ethan The Hubble Space Telescope, shut down for months now, came back online earlier this week and looks to be working just fine again. Check out its latest picture and see for yourself!

But I’m going to be running around in my Halloween Costume today; pictures tomorrow, I promise. In the meantime enjoy some scary Halloween Astronomy pictures, courtesy of the Hubble Space Telescope.

First up, we have eyeballs, starting with the Cat’s Eye Nebula:

Then there’s NGC 6751, which for some reason isn’t called the Eyeball Nebula:

And now we turn to astro-ghosts, with the Ghost Head Nebula (NGC 2080):

And finally, a little challenge! Can you spot the monster in this picture?

Happy Halloween!

Update: And if you get a chance this weekend, I’ve just been informed that two interesting shows are premiering on the National Geographic channel this weekend: Five Years On Mars this Sunday at 8 PM (about the Mars rovers) and Calling All Aliens after that at 10 PM (about SETI). Check it out if you’ve got time; they sound awesome!

Seriously, WTF is that? Fri, 19 Sep 2008 21:49:00 +0000 ethan I can usually rely on SWAB reader Dave to regularly ask some interesting questions. But he sent me a link to an article entitled, “Hubble Finds Unidentified Object in Space, Scientists Puzzled.” And my first thought is, of course, yeah, right.

But then I took a look at the article, and it turns out that we really are puzzled. Here’s the deal. While looking at a previously viewed patch of the sky, astronomers found a light source where there wasn’t one before. It continued to brighten, and for 100 days got brighter and brighter, eventually reaching a maximum. Then it dimmed out over the same time, and now there’s nothing again.

What’s the big deal about this? A number of things. First off, there’s nothing there. Nothing. No galaxies, no star clusters, no visible stars. Nothing. So either it’s something we’ve never seen before happening right here in our own galaxy, or there’s something going on so far away that we can’t even see the galaxy that’s hosting it with the Hubble Space Telescope! That’s right, Hubble Space Telescope: still a badass after all these years.

So we don’t see anything at all in the sky where this bright object appeared and then disappeared. Well, my friends, what do you suppose it was? Let’s examine the options:

  1. A Variable Star: Sure, why not? After all, there’s such a huge variety of variable stars that maybe one of them could brighten so much that we could see it only at its very brightest, and then it could disappear again, possibly for as long as 3 years or so. But this object’s spectrum doesn’t match up with variable stars.
  2. Nova or supernova: Yeah, sure, why not? They appear out of nowhere, get really bright, and then fade away. The problem is that this thing brightened more slowly than all known supernova types, and the spectrum not only doesn’t match novae or supernovae, but doesn’t match any of the spectra in the entire SDSS database, which has surveyed 10,000 square degrees on the sky! By comparison, this image, which shows a supernova going off in galaxy NGC 4526, shows about 0.02 square degrees of the sky.
  3. Microlensing: Alright, it must be microlensing. This is where a dark object moves in front of a bright one, increasing the amount of light that you see while everything is in perfect alignment, much like everyone’s favorite photoshop effect: the lens flare! The problem with this? The shape of the light curve is all wrong. See for yourself: on page two of this paper, the light-curve is way too broad to match up with microlensing, which looks like this:

So what does all of this mean? We’ve discovered something unlike anything ever recorded in the history of astronomy. My best guess? That it’s a new class of lensed object, and that if we perform an extra-deep telescopic survey of that area, we’ll find out exactly what it was that caused it. But for right now, it’ll have to remain a mystery!

Where are stars born? Fri, 15 Aug 2008 20:02:25 +0000 ethan No, it isn’t the Mickey Mouse Club; there are special places in space where stars can form. We call them star forming regions, and they’re some of the prettiest objects in the sky, looking something like this:

What are these places, and why do they look like this? These are giant clouds of gas, made of molecules like hydrogen, carbon dioxide, water, ammonia, and other gasses, but mostly they’re made of hydrogen. It took the Hubble Space Telescope to show us just how beautiful and varied these places are, and since it just celebrated it’s 100,000th orbit around Earth, here are a couple of the prettiest pictures it’s taken of these regions:

and a famous one in the Eagle Nebula:

Other than the amazing colors in the images, notice the big, obscuring clouds of what looks like dust? In the Eagle Nebula, they probably look more like pillars. These are regions where the gas is very dense, and where we think new stars are just starting to form. The way this works is that these big, diffuse clouds of gas start to collapse under their own gravity, and at some point they become so dense that they block the light coming from the stars behind and around them. Rather than being illuminated by them, like the rest of the nebula, they appear dark.

But when the density gets high enough, and the pressure of the surrounding gas gets large enough, nuclear fusion starts in the center, and that’s how you begin to form a star:

Some of the dust gets blown off, and some of it continues to accrete onto the proto-star. But the part you care about is some of that dust settles into a disk or a series of rings around the proto-star, and that eventually forms up into planets. We can actually see this in action around some stars:

And that’s not only where stars are born, but that’s where whole solar systems come from!

Can’t get enough of astronomy and outer space stuff? Check out this week’s Carnival of Space over at Next Generation, and enjoy!

Hubble Snaps History Tue, 10 Jun 2008 16:38:32 +0000 ethan In 1933, an astronomer named Fritz Zwicky took a good, long look at the largest cluster of galaxies known at the time, the Coma Cluster. It is still one of the largest and densest clusters of galaxies, with thousands and thousands of galaxies. Other people had noticed this cluster before, and in particular they noticed how, bizarrely, almost all of the galaxies in it were elliptical galaxies (left), rather than spirals (right). (We now know that this is true for all clusters, that they are rich in ellipticals.)

Fritz figured out how far apart these galaxies were, estimated their masses based on their starlight, and calculated how fast they ought to be moving due to Newton’s laws of gravity. And then he measured how fast they were moving, and found something interesting. Either the laws of gravity were way wrong, or there was a ton of matter that was dark! This was 65 years ago; people didn’t take dark matter seriously until over 40 years later. But Fritz was right, and the evidence for dark matter is now considered to be overwhelming.

The Coma Cluster is still an interesting and beautiful place, though, and now the Hubble Space Telescope has taken some amazing shots of it. This isn’t aimed at the center of the cluster, but about a third of the way out, about the same amount out that our Sun is from the center of the galaxy. Have a good look and enjoy the oohs and aahs, but don’t forget that this is the birthplace of our understanding of the dark side of the Universe!

There are a few spiral galaxies in the cluster; here’s one of the more spectacular ones:

But yeah, like I said, most of them are ellipticals. Really, much more than 50%. Take a look at the same size patch of sky, just with the camera pointed elsewhere:

And finally, to reward your patience, here is a full-scale image of the entire Coma Cluster, with major galaxies labeled, in as big a box as the screen will allow (click here to see the entire image):

How lonely are we, with just our one large galaxy, Andromeda, to call our neighbor?

Quasars: Worth a million bucks! Fri, 30 May 2008 20:50:58 +0000 ethan What would you do if you were a radio astronomer, looking up at the night sky with your giant radio telescope, and you saw a bunch of powerful radio waves being emitted from one point? Well, you’d point your other telescopes that are sensitive to visible light at it and see what the big idea was.

Sure, sometimes you’d see a galaxy or star cluster or planetary nebula, because those can emit radio waves. But sometimes you’d see just a little star-like dot in the sky, and sometimes you’d see nothing at all in the direction of the radio waves:

Well, what would you call such an object? The scientists who saw them called them Quasi-Stellar Radio Sources. QSRS, they wrote down. How do you pronounce that? Oh right, quasars! (See the wikipedia article for a false etymology.)

So what are these things? Well, we know from measuring their redshifts that they’re very far away, and from measuring their magnitudes that they’re extremely bright. From the emission lines, we can measure their masses, too; they typically weigh in at a few hundred million times that of our Sun. Occasionally, we get lucky, and can detect that they emit jets like quasar 3C 273:

So what makes them? Well, we now know that they’re collapsed clouds of matter that surround extremely massive black holes! The spinning black holes accelerate the matter and cause the emission of lots of different types of light, including radio waves, and we observe them. Close up, a quasar looks like this (the image is of quasar 3C 120, as taken by the Hubble Space Telescope):

Well, this year’s Kavli Prize for astrophysics goes to Maarten Schmidt and Donald Lynden-Bell for their discoveries that quasars are both so far away and so energetic, and how they get to be that way. And the award comes with a hefty cash sum, too. How much, you ask?

One Million Dollars! Have a great weekend, folks, and don’t forget to check out this week’s Carnival of Space, with a ton of articles about Mars and the Phoenix mission, as well as a special tribute to the NASA astronauts who lost their lives as part of the space program.

Do all Stars Eventually Explode? Mon, 07 Apr 2008 16:47:38 +0000 ethan What’s going to happen to all the stars in the Universe as they get older? Well, just as nothing can live forever, stars can’t live forever also. Why? Because they run on fuel: burning hydrogen into helium, for example. When they run out of fuel, something’s gotta give. Barbara Ryden reminds us of an excellent and appropriate quote by Dylan Thomas:

Do not go gentle into that good night.
Rage, rage against the dying of the light.

But what exactly happens to the star depends very sensitively on what the mass of the star is.

If you’ve got a tiny little star, less than about 40% of the mass of our Sun, it burns hydrogen to helium, and doesn’t have enough mass to burn helium any further. Our Sun will be able to burn helium into Carbon and Oxygen, and stars significantly more massive will be able to burn Carbon and Oxygen into Neon, Silicon, and even more massive stars will eventually burn those into Iron. Most stars that fall into this category, when they run out of fuel that they’re able to burn, will expand into a giant star, and then contract into a white dwarf.

White dwarfs don’t burn anything, and are only “white” because they emit light due to the energy released from contracting gravitationally. (For a size comparison, the radius of Earth is 6371 km.) When they’re done shrinking after a few billion more years, they stop emitting light, and become known as black dwarfs. But the most massive stars with iron cores, about 8 times the mass of the Sun or more, go supernova. When they start to contract, the pressure on the iron core becomes so intense that it starts to fuse the protons and electrons found in the iron atoms into neutrons! This causes a tremendous release of energy, known as a supernova explosion:

If the star is even more massive, the supernova can become even more powerful, and is known as a hypernova. Perhaps the most massive stars, dozens or even a hundred times as massive as our Sun, will go hypernova.

But maybe not! Astronomers from Ireland have been tracking exploded stars with the Hubble Space Telescope. They’ve been trying to determine what the masses of stars were before they went supernova, by trying to identify which star it was that exploded. Based on their findings, they’ve found that some stars may be massive enough that they don’t go supernova or hypernova, but instead, when they stop burning their fuel, collapse directly into a black hole! Now this is neat, because in theory it isn’t the most massive stars that do this, since their radii will be large enough that they won’t directly collapse into black holes, but some special mass range. Here’s a diagram I’ve found to illustrate the different possible fates of stars, based on their starting mass:

Now, what I want to know is, will the most massive stars in our galaxy, Eta Carinae, the Pistol Star, and LBV 1806-20, go hypernova when they die, or collapse directly to black holes? It might depend on where they are relative to the Eddington Limit, but for all we’re sure of at this point, it might as well depend on whether they’ve read their Dylan Thomas or not!

]]> Why we need Dark Matter in the Universe. Tue, 19 Feb 2008 20:20:55 +0000 ethan Last week, Pamela Gay over at Star Stryder pointed me to a press release which claimed that, among other things, perhaps dark matter wasn’t necessary. So I wrote a guest post on her blog explaining why it was. Apparently, some people still aren’t convinced. So I will lay out for you all the reasons I can think of why we need it, and explain what happens if you try to do without it.

1. Cluster Velocity Dispersions. When we take a look at galaxies, we often find hundreds or even thousands of them clustered together, like in the Coma Cluster (at left). We can measure how quickly those galaxies are moving around their center, and we can determine, based on the laws of gravity, how much mass is in these clusters. When we do these calculations, we find that Ωm, or the amount of total matter in the Universe, is about 20 to 30% of its critical value. (Note: Ωb, or the amount of matter in protons, neutrons, and electrons, is known from Big Bang Nucleosynthesis to be between only 4 and 5%.)

2. The Power Spectrum of the Universe. We can measure how many galaxies there are at a bunch of different positions in the Universe. There are big surveys like the 2-degree Field Galaxy Redshift Survey and the Sloan Digital Sky Survey, which have allowed us to construct maps of where all the galaxies in a large volume of the Universe are (see the image at right, where each little dot represents a galaxy). We can then determine, based on the clustering of these galaxies, how much total matterm) and how much normal matterb) there is. We find that Ωm is about 23% and Ωb is about 3 to 4%.

3. The Cosmic Microwave Background. The leftover radiation from the big bang is about 2.725 degrees Kelvin, and it’s approximately that temperature everywhere. But some spots are just a tiny bit hotter or colder (on the order of 10 microKelvins). We can learn a lot about cosmology from these temperature anisotropies, and one of them is how much dark matter and how much normal matter we have in the Universe. What we find from this is that the amount of total matter is about 27% and the amount of normal matter is 4-5%.

4. Gravitational Lensing Data. Again, let’s come back to my favorite nail-in-the-coffin of “we can just change the laws of gravity and do away with dark matter” theories: colliding galaxy clusters. There’s the Bullet Cluster,

there’s Abell 520,

and the cluster CL0024+17,

and the important thing is that we find that we have matter exerting gravity where no normal matter exists. Even if you fine-tune a theory of gravity to work for one of these three geometries, it doesn’t work for the other two. I will sum this up: there is no theory of modified gravity that can explain these three observations without also including dark matter.

5. Rotation of spiral galaxies. When a spiral galaxy rotates, we would expect, if there were no dark matter, for the outer parts to revolve around the center with a much slower velocity than the inner parts, just as the outer planets of our solar system revolve around the Sun with slower velocities than the inner planets. But we don’t see that. In fact, as far as we can tell, the velocities of rotating galaxies remain constant no matter how far out you go (see the image at right). This means either the laws of gravity are wrong for galaxies or there is some extra mass, like dark matter. But if you think the laws of gravity are wrong, I urge you to reread point number 4.

I realize this is especially confusing given the number of crackpots out there, who ignore some of these points and then claim we don’t need dark matter. I realize that there are alternative theories that can explain *some* of these theories, and there may even be alternatives to General Relativity like Bekenstein’s Tensor-Vector-Scalar gravity that can possibly explain points 1, 2, 3, and 5 without dark matter. But point 4 is really the killer. You want there to be gravity where there’s no matter? They haven’t made a theory that can do that in a way that’s consistent with observations, and people have been trying.

There are other tests that provide evidence for dark matter as well, but I think the five above should convince any skeptic of the facts that

  • Dark Matter explains all of these observations, while
  • modifying gravity cannot explain all of these observations.

Are you convinced? Leave me a comment about it. Still not convinced? Leave me a comment and tell me why!

An amazing discovery: a double Einstein Ring! Thu, 24 Jan 2008 21:22:16 +0000 ethan One of the perks of being a postdoc at a place like the University of Arizona, one of the top places in the US for astronomy, is that we get a number of really interesting visitors. Today we got paid a visit by Tommaso Treu, an astronomer at UC Santa Barbara.

He spoke to us today about one of his most recent, most interesting discoveries, done with the Hubble Space Telescope, of a double Einstein Ring. Take a look at the image below:
Double Einstein Ring
You’ll notice that there’s a ball of light in the center with some ring-like structure(s) around it. These things are rare, first off. There are only two ways that we know of to make something that looks like a ring in space:

  1. When a galaxy is being gravitationally captured by another galaxy, tidal forces can cause the captured galaxy to stretch apart into a ring around the larger galaxy. There are other gravitational methods of creating a ring (e.g., the rings around Saturn), but they don’t operate on scales as large as a galaxy. In these cases, the ring and the galaxy will be roughly the same distance away from us.
  2. The galaxies can be perfectly lined up with our own, making a three-in-a-row alignment. The galaxy farthest from us will be “lensed” by the one in the middle; the gravity from the intervening galaxy will bend the light into a ring if the alignment is perfect. This illustration shows how it happens:
    Einstein Rings

    In this case, the central galaxy and the ring around it will be at significantly different distances from one another.

So how can we tell which one is going on? We can tell whether the ring is part of the galaxy or whether it’s an Einstein Ring by measuring the redshifts (and hence the distances) of these objects. Let’s zoom in for a closer look and block the light from the central galaxy with a filter, and see what we can learn:
Zoomed in rings
Do you see how there’s not just one ring around the galaxy, but a fainter set of arcs in a ring-shape outside of it? This means that there aren’t two galaxies aligned with our own, but THREE, where the foreground galaxy is about 900 Megaparsecs from us, the “inner ring” is a galaxy about 1800 Megaparsecs away, and the “outer ring” is a galaxy about 3300 Megaparsecs away. (Remember that a Megaparsec is about 3.3 million light-years!) If it weren’t for the foreground galaxy, which not only stretches but magnifies those in back of it, we’d never see those background galaxies at all, because they’re small and faint, and so far away!

This alignment is so rare that it’s estimated there are less than 100 of these in the entire universe, and we just found one! (In fact, it’s very unlikely that there is even one “triple Einstein ring” in the entire universe.) What’s more, is that we can use these lenses, which are sensitive to the total mass of the objects, to learn about the dark matter in the galaxies; again, this is more evidence that is consistent with dark matter and inconsistent with modified gravity. In any case, this is rare and exciting stuff, and makes me excited that we still have the Hubble space telescope!