National Geographic News
A photo of the milky way

A dozen and a half dwarf galaxies orbit the mighty Milky Way, including tiny Segue 1 and Segue 2, with about a thousand stars apiece.


Michael Lemonick

for National Geographic


A tiny galaxy lying just beyond the edge of the Milky Way has failed to evolve normally over the last several billion years, say astronomers, and it represents a sample of the universe as it was in its infancy, around 13 billion years ago.

The small, faint galaxy known as Segue 1 may thus be a sort of fossil, which scientists can study to understand the nature of the earliest structures in the universe, researchers reported in the May 1 Astrophysical Journal.

"People have speculated about what those first galaxies might have looked like," said the paper's lead author, MIT astronomer Anna Frebel. Segue 1, she explained, is "like a cosmic Rosetta Stone" that will allow astronomers to trace the evolution of galaxies from the very beginning to the present day.

Astronomers already had good theoretical reasons to believe that the earliest galaxies in the universe were puny little things, far smaller and less massive than the Milky Way and its modern kin. Over billions of years, say the theorists, gravity forced these small clumps of stars and gas to merge together. Eventually they assembled themselves into the majestic spirals and bulbous elliptical galaxies we see today. (See "Photo Gallery: Galaxies.")

Thanks to all of that merging, nearly all of these primordial mini-galaxies have long since been dismembered. But a few remain, including Segue 1.

How Segue 1 Was Unmasked

Segue 1, discovered by the Sloan Digital Sky Survey in 2006, is one of the smallest galaxies ever found. The galaxy has only about 1,000 stars, compared with the Milky Way's 100 billion or more. But it's not the size that convinced Frebel and her colleagues that Segue 1 was so ancient—it's the composition of its stars.

Unlike the sun, the stars in Segue 1 are notably deficient in elements heavier than hydrogen and helium. That means that the gas clouds out of which the stars were born were also deficient in these elements.

That's an important clue. The very first stars in the cosmos were made almost entirely of hydrogen and helium, the only elements to emerge in any quantity from the big bang. Thermonuclear reactions in the cores of these stars forged the hydrogen and helium into heavier atoms, such as as carbon, nitrogen, and oxygen. When those stars exploded as supernovae, they enriched the surrounding gas with the heavy atoms, which were then incorporated into a second generation of stars.

The new stars forged even heavier elements, such as aluminum and iron, and exploded in turn, creating a new supply of element-rich gas for making stars like the sun—a third-generation star that was born about 4.6 billion years ago.

Galactic Suburbia

Photo of a train loaded with coal.
Star-poor Segue 1, orbiting the Milky Way on its fringe, has been called "a cosmic Rosetta Stone" to understanding the evolution of galaxies.

But observations with the powerful Keck I telescope, in Hawaii, and the Magellan and Very Large telescopes, in Chile, found no evidence that Segue 1 is home to any third-generation stars. "It may have tried to make them," said Frebel, "but for some reason it failed."

The best guess, according to Volker Bromm, a theorist at the University of Texas, Austin, who wasn't involved in the new study, is that at least some of Segue 1's second-generation stars did explode, but the element-rich gas clouds they created escaped the tiny galaxy's weak gravitational pull and puffed off into deep space. "There was no chance for another round of star formation," he said. "It was a kind of suicide."

That makes Segue 1 unique. It's not the only dwarf galaxy that orbits the Milky Way—there are more than a dozen of them. But of the others studied so far, all are massive enough to have held onto their gas and formed third-generation stars. When that happens, said Bromm, the story of chemical evolution has too many layers. "It's very difficult," he said, "to disentangle the message" and understand which stars belong to which generation—or even whether a particular star might have formed from a mix of first- and second-generation gas clouds.

Frebel is already on the search for more galaxies like Segue 1, which may lead to insights into the early universe. But for now, Segue 1 appears to be an outlier. "For some reason, this little guy has managed to survive for 13 billion years," said Frebel. "It's truly amazing."


Siegfried Neumann
Siegfried Neumann

Equally it would be amazing to know where all the chemical elements of the periodic system we can find on earth come from and how they have been generated or breeded.

Gerry Vankoughnett
Gerry Vankoughnett

I think Doc Brown needs to pay Segue 1 a visit with his Mr. Fusion Home Energy Reactor and the flux capacitor to give a boost.

Francesco Sinibaldi
Francesco Sinibaldi

Stream of memories.

Beside that flower

the whisper of

a delicate ray

touches the margin

described by the

first light: there,

in the garden,

near the youth

of a fine rose.

Francesco Sinibaldi

Thomas Youngblood
Thomas Youngblood

What I always understood about stars is that as they age they turn their elements into heavier elements. I never heard of a reason for a star to die before creating heavy elements regardless of the material it started with. 

If these stars all have light elements, doesn't that make them young stars by default? 

Rather, it is the source of their starting elements that makes the difference. Thus, my question is: why and how did a cloud of hydrogen and helium manage to come to be where it was when these stars began to form?

I'm thinking that could mean that the gas remained pure all the time from the big bang before finally gathering close enough to create stars. Any ideas?

Mia O' Connel
Mia O' Connel

Interesting article, but I don't understand it. They talk like evolution is a guaranteed thing, that the universe is billions of years old, and that stuff evolves. But none of that is proven by science, most doesn't even have a lot of scientific evidence supporting it. I just can't understand how so many smart people refuse to see what is right in front of them. They act like there's no question about the authenticity of an ancient universe or evolution.

Eduardo Santilli
Eduardo Santilli

I didn't even know there were a dozen and a half satellite galaxies orbiting our own milky way. It´s amazing what scientists are discovering every day.

The "empty space" is far beyond of being really empty.

Who has made all of this and why?!

Jason Rutter
Jason Rutter

I am not a physicist either, but I think that after the big bang there was so much energy only sub-atomic particles existed. Then everything cooled as it expanded and during that cooling more complex things formed, like hydrogen. The hydrogen coalesced into gas clouds, then into stars. in stars, you can make all the elements below Iron without a supernova. You need a supernova to make the elements above iron. Like I said, I'm not a physicist so don't give me a hard time if this isn't 100%.

Mike Barrett
Mike Barrett

Hmmm, so the big bang didn't have enough energy to populate the universe w/ heavy atoms.  I am no quantum/astrophysicist but... the assumption that it didn't begs a question.  Sometime after the big bang - energy levels had to be at the right level to create heavy atoms from what ever matter was around (H or He, etc?).   

Not that anyone would because I asked, but there should be some sort of refutation of this postulate somewhere in the academic history of the big bang theory.

Justin L.
Justin L.

@Mia O' Connel  There is no question about the certainty and authenticity about evolution.  It has been proven by science, without a doubt.  You can witness it should you chose to educate yourself and take a trip to the Galapagos Islands and see how evolution has changed the creatures that reside there.  You can also chose to remain ignorant and continue to accept superstition, believing that some mystical being put similar yet unique creatures on a few neighboring islands, and that those creatures have remained unchanged since the earth was created 10,000 years ago (or whatever age you chose to believe this rock in the center of the universe is.)  It's a free country...

Todd Brown
Todd Brown

@Mia O' Connel All the scientific evidence points in one direction.  The red shifting of light correlates very well to distances of galaxies.  The ratios of radioactive elements in rocks again correlates to when the stone was solidified from lava.  The temperature of the cosmic microwave background also suggests the same time frame for the cooling as the universe expands.

The universe is roughly 13 billion years old and the sun and earth are about 4.5 billion years old.  It is over 60 years since the last alternative theory, the Steady State theory was shown to not fit the data.

I must be blind, please tell me what is in front of me that I do not see.

Todd Brown
Todd Brown

@Mike Barrett Not wanting to get too technical, the heaviest elements that could be made during the Big Bang were Lithium and Beryllium of atomic weight 7.  Nothing heavier was made because there were no species of atomic weight 5 or 8 available to synthesis heavier elements.  As it was not that much Helium4 was made because He4 is made from two deuterium ( hydrogen with one proton and one neutron) because it was so hot that photons of light had energy greater that the binding energy of deuterium.  So as soon as deuterium formed it was immediately broken back down to a free proton and a free neutron.

So first generation stars are almost nothing but Hydrogen and Helium.  Stars as they near the end of their life cycle start fusing elements up to iron.  But fusing iron takes in energy, not giving energy off.  All the heavier elements only form during the explosion of a star.

Jeffrey Allan
Jeffrey Allan

Hey @Mike Barrett . I think that the most popular theory these days is that all of this H and He formed large stars and that though the nuclear fusion these stars created heavier elements and then when these stars went super nova it sent this material out in to the universe. 


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