Galaxies come in a lot of shapes and sizes: huge ellipticals, big spirals, weird squishy irregulars. There is a sub-class called "dwarf galaxies" which are smaller than usual. We actually think they dominate the Universe by number, but because they have fewer stars – a few billion or so tops, compared to the hundreds of billions of a big one like our Milky Way – they fade rapidly with distance. Only a handful are close enough to study well.
One of these is DDO 190, a nice little dude something like 9 million light years away. That’s close enough to resolve individual stars in the galaxy, as you can see in this really pretty Hubble image of it:
[Click to galactinate, or grab the cosmic 3700 x 2600 pixel version.]
DDO 190 is small, but not tiny: about 15,000 light years across. That’s about 1/6th the size of our galaxy. It’s also well outside our Local group of nearby galaxies (the Andromeda galaxy is less than 3 million light years distant from us, for comparison) and is thought to be part of the M94 galaxy group. But if true it’s fairly isolated even from the others on its team; the nearest neighbor appears to be another dwarf galaxy several million light years away from it.
This image is pretty nifty. For one thing, you can see lots of far more distant background galaxies, some right through DDO 190, which always gives me a kick. But the dwarf galaxy itself has some surprises. The bluish fuzzy regions are clouds of gas lit by young, hot stars. These stars don’t live long (a few million years or so), meaning there’s still some star birth going on in the little guy. That blue patch at the bottom is the brightest of them – it looks a bit like a more distant galaxy, but don’t be fooled.
Interestingly, it has two different populations of stars in it. The younger ones I mentioned (100 million years or younger) tend to be close in to the center, while older ones (4 billion years or more) are located in the outskirts. This is common in dwarf irregular galaxies. The older stars may be showing us what the primeval galaxy looked like, but now a burst of star birth has occurred near the center, making the galaxy look more condensed.
Since the vast majority of galaxies in the Universe are dwarfs like this, we think bigger ones like ours get to their size by gravitationally colliding with and absorbing dwarfs. In fact, we know the Milky Way is eating several right now!
Galaxies are cool, and pretty, and magnificent, but they’re also cannibals. DDO 190 is isolated enough that it may be safe from that fate for quite some time. But the Universe is young, and galaxies patient. In a trillion years or so, we’ll see who has whom over for dinner.
- And the cottonball galaxies shall inherit the Universe
- Hubble grills a confused galaxy
- Obese, gluttonous, and cannibalistic is no way to go through life, son
- Lonely galaxy is lonely. But it ate its friends.
[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I'm passing on to you! You're welcome.]
Today’s BAFact: How much brighter is the Sun than the faintest object ever seen? About Avogadro’s number times brighter.
Yesterday and the day before I wrote about how much brighter the Sun is than the Moon, and how much brighter the Sun is than the faintest star you can see (note that here I mean apparent brightness, that is, how bright it is in the sky, not how luminous it actually is). I have one more thing to add here.
Years ago, I worked on a Hubble Space Telescope camera called STIS – the Space Telescope Imaging Spectrograph. At the time, it was the most sensitive camera ever flown in space, and I was constantly amazed at what we saw using it.
Hubble did a series of observations called the Deep Fields: it stared at one spot in the sky for days, letting light from incredibly faint objects build up so that they could be detected. For the Deep Field South, STIS was used to observe a particular kind of galaxy, a quasar called J2233-606. The total observation time was over 150,000 seconds – nearly two days!
I worked on these images, and was chatting with a friend about them. We were astonished at the number of objects we could see, distant galaxies so faint that they were unnamed, uncategorized, because no one had ever seen them before. Playing with the numbers, we figured that the faintest objects we could see in the observations had a magnitude of about 31.5. That’s incredibly faint.
How faint, exactly?
The faintest star you can see with just your eye has a magnitude of about 6. Using the magnitude equation I wrote about earlier, plugging those numbers in we get
Brightness ratio = 2.512(31.5 – 6)) = 2.51225.5 = 16 billion
But we can do better than that. A lot better. After all, the Sun is the brightest object in the sky, of course, with a magnitude of -26.7. Just for grins, how much brighter is the Sun than the faintest objects ever seen?
Brightness ratio = 2.512(31.5 – (-26.7)) = 2.51258.2 = 2 x 1023
That’s 200,000,000,000,000,000,000,000. 200 sextillion. Holy yikes.
That number is crushing my mind. It’s ridiculous. A sextillion is simply too big a number to grasp. And 200 of them? C’mon!
But hey, wait a sec…
Does the number 2 x 1023 look familiar to you? It does to me: it’s the same order of magnitude (factor of 10) as Avogadro’s number! It’s the number of atoms of an element in a mole of the element, where a mole is the number of atoms in 12 grams of pure carbon-12. I know, it’s an odd unit, but it’s handy in chemistry, and a lot of (geeky) folks have heard of it.
Avogadro’s number is actually about 6 x 1023. So if we could detect stars or galaxies just a hair more than a magnitude fainter, the ratio of the brightness of the Sun to those objects would be Avogadro’s number. Huh.
I’m not sure that helps, but it’s fun in a spectacularly nerdtastic kind of way.
Science, baby. I love this stuff!
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- Announcing BAFacts: a daily dose of sciencey fun
It’s been a while since I posted a devastating spiral galaxy picture, so why not now? I present NGC 1187, a gorgeous barred spiral about 60 million light years away:
[Click to galactinate, or grab the 3300 x 1900 pixel version.]
This image was taken with the Very Large Telescope to help astronomers study a star called Supernova 2007Y that exploded in this galaxy. Can you find it? Yeah, good luck with that; there are a gazillion stars in the picture. The folks at the European Southern Observatory helpfully circled it in this annotated version. Were you right?*
Anyway, this was the second supernova in NGC 1187 in recent times; it also hosted one in 1982 (which had faded into obscurity by the time the above image was taken). Most spirals have supernovae in them every century or three, so this was unusual but not necessarily weird. The rate is statistical so you might get two close together, or a long stretch without one. The last one in our Milky Way was about 170 years ago, and the last known before that was 400 years ago.
NGC 1187 is a gas-rich galaxy, and is forming lots of stars. That might lead to a higher-than-normal supernova rate, since that means more high-mass stars are being born, only to explode a few million years later. Both of the recent supernovae in NGC 1187 were of the same type – the core collapse of a high-mass star – so maybe this does play into it. I suppose time will tell. If we get an elevated rate over the next few decades, that’ll be interesting.
Astronomers will of course continue to study this galaxy and look for more supernovae. The science of that would be well worth the time, and in the meantime we would get even more lovely pictures of this spectacular island universe.
Image credit: ESO
It’s generally said that discoveries in science tend to be at the thin hairy edge of what you can do — always at the faintest limits you can see, the furthest reaches, the lowest signals. That can be trivially true because stuff that’s easy to find has already been discovered. But many times, when you’re looking farther and fainter than you ever have, you find things that really are new… and can (maybe!) be a problem for existing models of how the Universe behaves.
Astronomers ran across just such thing recently. Hubble observations of a distant galaxy cluster revealed an arc of light above it. That’s actually the distorted image of a more distant galaxy, and it’s a common enough sight near foreground clusters. But the thing is, that galaxy shouldn’t be there.
This picture is a combination of two images taken in the near-infrared using Hubble. The cluster is the clump of fuzzy blobs in the center left. The small square outlines the arc, and the big square zooms in on it.
The cluster is unusual. It’s at a distance of nearly 10 billion light years away. Clusters have been seen that far away, so by itself that’s not so odd. The thing is, it’s a whopper: the total mass in all those galaxies combined may be as much as a staggering 500 trillion times the mass of the Sun, making this by far the most massive cluster seen at that distance.
But that arc… First, things like this are seen pretty often near clusters. They’re gravitational lenses: the gravity from the cluster bends the light from a more distant galaxy in the background, bending its shape into an arc. See Related Posts below for lots of info and cool pictures on these arcs. In this case, I’ll note the shape of the arc implies the biggest galaxy in the cluster, the one right below the small square, is doing most of the lensing.
But here’s the problem: the galaxy whose light is getting bent has to be on the other side of the cluster, and that cluster is really far away. Note only that, the galaxy has to be bright enough that we can see it at all. Combined, this should make an arc like this rare. Really rare.
So rare, in fact, that it shouldn’t be there at all! The astronomers who did this research worked through the physics and statistics, and what they found is that the odds of seeing this arc in this way are zero. As in, what the heck is it doing there at all?
You’d think that with all our fancy equipment and technology, all the nearby galaxies in the Universe would’ve been spotted by now. But it turns out that’s not the case. Some galaxies are very faint — small, with few stars — making them tough to find even when relatively speaking they’re in our neighborhood.
So say hello to our newly-discovered neighbor, UGC 4597!
[Click to galactinate.]
UGC 4597 is a dwarf galaxy. Galaxies like our Milky Way have billions or hundreds of billions of stars, but dwarf galaxies have stars numbering in the millions. That’s why it remained undiscovered until just a few years ago — it turned up in a survey taken in 2008! Astronomers were looking for dwarf companions to the splashy spiral galaxy M81 located about 12 million light years away, and dinky UGC 5497 showed up.
The image above was taken by Hubble in late 2009. Of course, in this shot it looks obvious enough, but this only shows a teeny portion of the sky. Because it’s so close to us, the entire M81 group of galaxies covers an area of the sky something like 20 times the size of the full Moon — thousands of times the size of this diminutive dwarf. That’s how it remained undiscovered for so long.
The image is a combination of two separates shots, one in visible light and one in near-infrared. The stars look very blue, with very few being red. Without a third image taken in bluer light it’s hard to be completely sure, but the color here most likely means that most of these stars are young, created in a wave of star formation a few million years ago. Just above and to the right of center of the core of the galaxy is a reddish patch; I thought initially that might be a gas cloud of some sort, but now I suspect it’s a background galaxy. In the full-res version of the picture you can see dozens of distant galaxies littering the scene, typical for a Hubble picture. They’re most likely hundreds of millions and even billions of light years away, far, far in the background.
That bright star on the right and the fainter one on the left are probably stars in the foreground, in our own galaxy. Sometimes that fact gets me even more than the rich science of the galaxies themselves: the depth of time and space we see in images like this. Nearby objects like local stars, medium-distance objects like neighborhood galaxies, and then mind-crushingly distant galaxies so far away that the light we see from them left when the newest evolutionary invention on Earth were organisms with more than one cell!
Astronomy may be all about looking out into the Universe, but it’s the perspective on ourselves that always stirs my mind.
ESA/Hubble & NASA
This is a galaxy?
Yup. It is! [Click to galactinate.]
This is the dinky Antlia Dwarf Galaxy (located in the southern constellation of Antlia, the "pump"), technically called a dwarf elliptical. It’s so faint and sparse that it wasn’t discovered until 1985 (and confirmed as being a galaxy in 1997), even though it’s only 4 million light years from Earth… not terribly farther than the Andromeda Galaxy, which is so big it’s visible to the naked eye! Antlia may be a member of the Local Group, a loose collection of a few dozen mostly small nearby galaxies; the Milky Way and Andromeda are the two biggest members.
This image is from Hubble, and shows just how dim a bulb this galaxy is. It only has a few million stars in it — our Milky Way has over a hundred billion, by comparison — and it’s only a few thousand light years across. The Milky Way is a full 100,000 light years in diameter, so if you put Antlia next to it you’d probably miss it entirely. Note that in this picture you’re only seeing the brightest stars in Antlia. At this distance, a star like the Sun in Antlia would be a tough object to see, even with Hubble. Most of the stars you see here are red giants, stars near the ends of their lives and thousands of times more luminous than the Sun.
But it’s an intriguing little bugger. For one thing, some of its stars are clearly very old, ten billion years or so. But other stars are just as clearly young, having been formed only a hundred million years ago or so (and I found a paper claiming it may have younger stars yet). That means Antlia has had more than one episode of star birth… but it doesn’t appear to be actively churning out stars now. If it did, the bright pinkish-red nebulae that form stars would be really obvious, especially in a galaxy this close by (like in, say, NGC 1427A).
Sometimes, I like to think of a photon of light as a car on a road. As the road dips and curves, a car has to follow that path, dipping and curving as well. It might be weird to think of space as curving, but it does. Gravity from massive objects warps space, and a beam of light moving through that curved space curves along with it.
This is the principle behind what’s called gravitational lensing. A beam of light passing by an object — a big galaxy, say, or a cluster of galaxies — bends one way. A beam headed in a slightly different direction bends a slightly different way. This can really mess with what we see… which I can prove! Check this out: a Hubble image of the galaxy RCSGA 032727-13260.
What a mess! All those arcs and blue smudges are images of that one galaxy. The light from that galaxy traveled nearly 10 billion light years to get here! But when it was halfway here, that light passed by the big cluster of galaxies — the red fuzzballs — in the middle of the image. As it did, the curvature of space distorted and warped the light from the galaxy, and by the time it reached us here at Earth the image looks like this. The outstretched, smeared-out arc is amazing; I’ve never seen one that long and well-defined before.
Not only that, but the image gets broken up into several separate images. There are no fewer than four different repetitions of the background galaxy in the big image. To show that, I put three of them together here. It’s goofed up, to be sure, but you can kinda sorta see they are the same galaxy, flipped over and/or smudged out.
The cool thing about this is we can learn about the more distant galaxy by examining these images. Read More
A very interesting set of observations has resulted in a conclusion that is somehow, paradoxically, both expected and startling: there are hundreds of billions of planets in our galaxy alone!
It’s expected because all the research being done for the past few years has been zeroing in on how many stars have planets, and it’s looking more and more like they’re very common. I’ll get into that in a sec. But it’s also startling, because HOLY COW THERE MAY BE HUNDREDS OF BILLIONS OF PLANETS IN OUR GALAXY ALONE!
Ahem. OK. So what’s going on here?
The new result comes from what’s called microlensing. The gravity of a star or planet can bend the light coming from an even more distant star, briefly magnifying it. The way the star light gets brighter over time can reveal the mass of the object doing the magnifying — the "lens", as it were. If a star passes in front of another star, you get a rise and then fall in the brightness, but if a planet is orbiting that nearer star, you get a second, smaller bump as well.
This kind of event takes an extraordinarily precise alignment, so they’re extremely rare. To compensate, you need to look at a lot of stars. So astronomers did: a survey using two telescopes covered several million stars every night, looking for the tell-tale bump(s). Over the course of six years, they found three — yes, only three — planets orbiting other stars acting like wee distant lenses. But that number is actually pretty good: when combined with previous surveys, and also taking into account how many lenses they didn’t see (which is important, statistically), they can extrapolate with some confidence about the numbers and types of exoplanets out there.
Their most basic result, and the one causing the stir, is that they find that there are likely hundreds of billions of planets orbiting other stars in our galaxy alone. Given that there are a few hundred billion stars in the Milky Way, this means on average there are about one or two planets per star in our galaxy! Now, let me be clear: this is an average. I’ve seen reports saying every star in our galaxy has a planet, and that’s not necessarily the case. You could have one star, say, with ten planets, and then nine with none and get the same results here.
The results get even more interesting when you break them down by planet type. Read More