Star Nearly As Old As Universe Found

The discovery of a dwarf galaxy star nearly as old as the universe has provided a missing link in supporting the “cannibalistic” theory of galaxy formation, according to a study led by Harvard researcher Anna Frebel that was published in the journal “Nature” this month.

For years astronomers have discussed the theory that the Milky Way formed by “eating up” smaller dwarf galaxies, but there was a crucial piece of evidence absent from such an explanation, said Frebel, who is a researcher at the Harvard-Smithsonian Center for Astrophysics.

“Previously, nobody had found any stars in the dwarf galaxies that looked very much like the most metal-poor stars in the Milky Way halo,” said Joshua D. Simon, a researcher at the Observatories of the Carnegie Institution of Washington and one of the study’s authors. “It was reasonable to wonder if maybe they didn’t exist.”

But after hours of stargazing and spectroscopic measurements, a star in a distant dwarf galaxy called the Sculptor Galaxy has proven to be chemically similar to several stars found in the Milky Way halo.

“It looks like many dwarf galaxies got shredded and deposited their stars in the halo of the Milky Way,” Frebel said.

Astronomers can determine how long ago a star was created by measuring its metal content because stars that contain more elements beyond hydrogen and helium are newer than “metal-poor” stars like the one found in the Sculptor Galaxy.

In our roughly 13.7 billion year old universe, Frebel said she estimated that the star in Sculptor was 12 to 13 billion years old.

“If we’re in middle age now, this star was born when the universe was basically a toddler,” said Evan N. Kirby, a researcher at California Institute of Technology and one of the authors.

According to Simon, previous methods of detecting metal-poor stars proved less useful the more metal-poor the star was.

“They kind of hit a floor in the metal abundance they would give you,” he said.

Finding a more accurate way to obtain these measurements was the subject of Kirby’s PhD thesis.

Using the Keck Telescope in Hawaii, Kirby set out to find evidence of metal-poor stars in dwarf galaxies by scanning the skies using medium-resolution spectroscopy, which allowed him to study many stars but didn’t yield a detail-rich characterization of any one star.

“I pointed the spectrometer at Sculptor and other galaxies, and out of about 400 stars in the sky I noticed this one,” he said.

He then offered the coordinates of this star to Frebel, who departed for Chile to take a closer look with the Magellen-Clay Telescope using high-resolution spectroscopy. Though this kind of spectroscopy takes much longer, it provides more details about the “personality” of the star, Frebel said.

She explained that gathering data for the star took eight to ten hours spanning two or three nights. A typical resolution, she added, usually takes one to two hours.

Besides time constraints, Frebel also had to keep an eye on the weather. Even mild cloud cover or slight winds can greatly interfere with nightly measurements, she said.

“We don’t want the stars to twinkle—we want them to stare at us straight without ever winking,” she explained.

Astronomy professor Abraham “Avi” Loeb, who is writing a book about how the first galaxies were created, called Frebel a pioneer, and said he believes this research puts the theory that the galaxy was formed out of small building blocks “on more robust ground.”

Frebel is back in Chile this week continuing the same line of research, hoping to find even more metal-poor stars in dwarf galaxies.

“Now that we’ve smelled blood we want to look out farther and observe fainter stars to test the merging theory in more detail,” she said.

—Staff writer Julie R. Barzilay can be reached at jbarzilay13@college.harvard.edu.