Meet the Itsy-Bitsy, Teeny-Weeny Galaxy

A massive swirl of stars like the Milky Way is not the only kind of galaxy out there — as a new pipsqueak member of the galactic community shows

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NASA / National Geographic / Getty Images

A bright blue newborn stars blast a hole through a nearby dwarf galaxy.

The late Carl Sagan got lots of praise for his brilliance in explaining science to the masses, especially on his PBS TV series Cosmos back in the 1980s. But he also got teased unmercifully for the super-geeky way he said “billions” — and given the subject matter, he said it a lot. The universe is billions of years old, and billions of light-years across, and contains a hundred billion galaxies, each containing a hundred billion stars…and Sagan got his teeth into every “billion” he could find.

Which is why he would be at something of a loss talking about a new report in The Astrophysical Journal describing the least massive galaxy every found. Known as Segue 2, it contains just 1,000 or so stars, and while this puny object, like most galaxies, is somewhat bulked up with invisible dark matter, that only adds another 100,000 stars’ worth. There’s not a billion to be found, unless you start weighing the stars and dark matter in pounds. “Finding a galaxy as tiny as Segue 2,” says co-author James Bullock, of the University of California, Irvine, “is like discovering an elephant smaller than a mouse.”

But it’s also very different. A mouse-sized elephant is the last thing you’d ever go looking for; a galaxy this tiny, by contrast, is something astronomers have long sought. The reason: it’s firmly established by now that most of the mass in the universe comes in the form of dark matter, not stars. The leading candidate for what makes up the dark matter is some sort of still-undiscovered subatomic particle. And computer models suggest that while these particles should coalesce into gigantic blobs or haloes that surround normal galaxies, they should  also form into thousands of much smaller clumps, buzzing around galactic fringes.

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Those smaller clumps would have their own tiny retinue of stars as well — and in 2006, astronomers finally found an example. Called Segue 1 (it was discovered by the Sloan Extension for Galactic Understanding and Exploration, or SEGUE, an offshoot of the Sloan Digital Sky Survey) it had a mere 300 or so stars, but its dark matter component was equivalent to about 600,000 stars — small, but not as small as what astronomers were hoping to find.

Then, in 2009, the SEGUE survey snagged a true pipsqueak: Segue 2 has three times the star count as its brother, but only one-sixth the dark matter — right in the ballpark of what theory says should be there. “It’s quite encouraging,” says Kirby.

Maybe, but it’s reasonable to wonder why you’d give a tiny thing like this a grandiose name such as “galaxy.” Why not just call it a star cluster? The answer, says lead author Evan Kirby, also at Irvine: a galaxy is defined as a system of stars that can enrich its own chemical composition. What that means is that the earliest stars were made almost entirely of hydrogen and helium. Those atoms were forged into heavier elements in the stars’ cores, and eventually blown out into space at high velocity in supernova explosions. The heavier elements, known as “metals,” then became incorporated into the next generation of stars (in astronomy jargon, iron and aluminum are metals, but so are oxygen and carbon).

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In a star cluster like the Pleiades, there isn’t enough gravity to keep those metals from flying away; in a galaxy, there is — and that gravity can come from either stars or dark matter, doesn’t matter which. Kirby and his team used the powerful Keck II telescope, in Hawaii, to measure the metal content of the stars in Segue 2, and found that they vary a lot. That’s evidence that some stars are younger, some older, and that the metals have stayed put. Puny or not, Segue 2 is  a galaxy. It’s also likely not to be the only one of its kind. “We think there are lots more,” says Kirby.

Unfortunately, those junior members of the galactic corps might not be easy to find. Spotting very dim collections of stars on the fringe of the Milky Way isn’t incredibly hard, but probing their light for evidence of metal composition, and measuring their orbital speeds as an indirect method of weighing their dark matter, is. The Keck II armed with an instrument called the DEIMOS spectrograph — among the world’s most powerful systems for doing this kind of work — can just manage it.

At this point, says Bullock, there’s no set of instruments, either in existence or on the drawing board, that could do a lot better. “Right now,” he says, “we’re thinking of what it would take.” Once the astronomers come up with something though, they’re betting that the little galaxies will be out there waiting.

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77 comments
a.gentleman49
a.gentleman49

We've got Harvard PhD's in my family and in my opinion, Carl Sagan was an idiot.

ElizabethMcBride-Lilleg
ElizabethMcBride-Lilleg

Cosmos is one of my favorite TV series of all time.  It was on when I was in early elementary school, and I absorbed every bit of it.  In first grade I was reading the book with my Dad before bedtime.  To this day, my view of the universe and my tiny place in it is strongly based on Carl Sagan's influence.  The wonders that the religious see as the gifts of God, I see as examples of the limitless possibilities and probabilities of the natural universe.

OccamsRazor1349
OccamsRazor1349

I love this stuff. Reminds me of how very insignificant we are in the grand scheme of things.

stefano3029
stefano3029

that's really cool that they're always discoveries new stuff out in space but the truth is everything that they are doing exploring space thus finding galaxies like segue 2 is amazing so small yet rich with beauty which is one of the reasons why astronomy is so interesting to do because in the process you learn amazing new things about the universe in which we live in so phenomenal

cheme911
cheme911

Guys,

I found the 95% dark/vacuum energy a year ago, where have you been? We call it our weather when actualy it is the decay of our quantum gravity field of energetic particles and strings from the sun in our atmosphere. It triggers vacuum condensing and vacuum evaporation and electromagnetic disturbances above our heads we call lightning. It is orbiting through our 5%.

darkmattersalot.com

DaveHinckley
DaveHinckley

I think Carl said "billions of billions", not "billions and billions".

RodVenger
RodVenger

They keep talking about 'dark matter' as if it's a fact and they come dangerously close to creating a pseudo-religion around it when they compose entire theories about how things work based on something they can't prove exists. Like as not, the 'missing matter' is bound up in black holes, whose actual mass they cannot determine, especially if much of that mass essentially exists outside of the Universe. My guess...the mass is in transit between holes, going in one, out the other, aka wormholes associated with the black holes.

texasghost01
texasghost01

He never said billions and billions...but he did say...billions a lot.

cpc65
cpc65

I've been watching episodes of Cosmos on Hulu lately. It was a great show. Some of the information is a bit dated as we learn more about the universe, but it's still great today.

Fun facts: Carl emphasized the B in billions to make sure it sounded distinct enough from millions so folks wouldn't get them mixed up. Also, just as nobody in Star Trek (tos) ever said, "Beam me up, Scotty!", Carl never said "Billions and billions" on Cosmos. That would be from Mr. Johnny Carson's great impersonations or Mr. Sagan on The Tonight Show. 

YoungbloodK
YoungbloodK

Wow, it touches my spirit to fathom the universe and how small we are as humans in the equation of life's vast cosmos.  I am always at awe with these types of pictures/articles to realize how insignificant and shabby we can behave as humans and miss understanding the majestic nature of the universe.  It seems we miss the point and squalor with trivial comments.  Humankind, humph!

MichaelJ.Arvizu
MichaelJ.Arvizu

It seems as if TIME has it in for Carl Sagan, based on just the first few sentences of this article.

CrystalShepard
CrystalShepard

Dr. Sagan didn't say "billions" in a super-geeky way.  The man had an accent.  I don't know whether the "super-geeky" comment shows ignorance of Dr. Sagan, a disdain for scientists, or a subtle form of racism.  Whatever it is, it's sad.

solex675
solex675

Seven comments in three hours. Quite a statement about our interest in science.

RuiPereira
RuiPereira

There is NO correlation between the theory of dark matter and the mass of this galaxy. Dark Matter is nothing more than a theory - with NO proof of any kind and ripe with conjuncture. The media is being taken in by theories that fill in the holes with made up concepts.

rrenegade
rrenegade

Typo:  least massive galaxy EVER found.  Not every found.

ScienceIsGood
ScienceIsGood

Why label a brilliant man such as Carl Sagan, one of the most brilliant men in human history, with the negatively connoted jibe "geeky"?

It does a disservice to people who take science and facts seriously. If you're going to write about science and scientists, don't apologize for being intellectual. If people are too stupid to pay attention or to get an education, that is THEIR fault.

mikehaggag
mikehaggag

Soon, tiny galaxies will be contained in small glass globes with falling snow flakes and sold at street corners by souvenirs merchants.