French astronomer Edouard Stephan discovered a group of enchanting galaxies in 1877 by using the first telescope to have a silver-backed glass mirror. In what is now known as Stephan's Quintet sit five galaxies within a Milky Way's length from one another, stretching out heavy arms of star clusters.
The undulating, interactive appearance of the group has encouraged astronomers to guess at the group's formative processes for more than a century. Now, another landmark telescope, the Hubble Space Telescope, with its ability to keenly differentiate between young, hot blue stars and older, cooler red stars, has finally brought the mysterious beginnings of Stephen's Quintet into full focus.
Hubble's ability to discern the stars' different ages confirms that the star clusters are much younger than the galaxies themselves, and were created by two separate galactic collisions.
"Knowing the colors allowed us to date the stars, and because some were born at the same time, it's a fairly clean way of measuring age," said Sarah Gallagher, a graduate student at Penn State and principal investigator of the project.
"[Stephen's Quintet] is a beautiful system for putting time on these events because in astronomy we usually just get a snapshot of what's been happening or what's going to happen, not such a timeline."
Crash and burn
The Quintet is made up of five galaxies numbered NGC 7317 through 7320 (there are two 7318s, A and B). More than 100 star clusters and several dwarf galaxies also reside within Stephen's Quintet, with the most remarkable amounts of non-galactic stars in the Tidal Tail of 7319 and in the northern Starburst Region that lies between galaxies 7318B and 7319. The ages of the stars in the five galaxies are nearly a billion years old, but the stars lying outside the galaxies are now known to be much younger.
And they lie in telltale trajectories.
"Hubble allows us to see a higher level of detail than ever before," said Jane Charlton a professor of astronomy and astrophysics at Penn State who directed the project with results published this week in the Astronomical Journal. "We know now that these [non-galactic] stars are so young that they had to have formed from interactions in the [recent] past."
One interaction discerned from the Hubble images was a zooming hit-and-run collision that happened 100 million years ago between the galactic gases of NGC 7319 and NGC 7320C, a galaxy no longer a part of the group. As the enormous star and gas clouds passed each other, the tail of the NGC 7319 galaxy was pulled out like a swath of cotton candy the length of the Milky Way.
NGC 7319's gases also were dumped into the Starburst Region from this collision. Both the tail and the Starburst Region now bulge with countless 100 million-year-old stars because of the increase in pressure and gravity of the crash.
But the Starburst Region also holds countless numbers of 20-million-year-old stars. These younger stars, the scientists believe, were formed when Galaxy NGC18B swooped in from behind and came crashing through the Starburst Region 20 million years ago. This collision triggered yet another round of intense star formation.
Disappearing evidence
Clusters of galaxies like the quintet are quite uncommon due to the Universe's continual expansion and vast expanses. But because this group insists on huddling under the shimmer of its new stars, said Gallagher, astronomers can study Stephan's Quintet as a sample of our earliest Universe.
Without Hubble and scientists rubber-necking at Stephen's Quintet, conditions of the earlier Universe may have remained a larger mystery than it is.
"In 200 million years from now we might not know that they interacted at all," Gallagher said. "We're basically looking at the collision now. It's right at the aftermath. If we came back in 200 million years, it would be like seeing the glass and smashed up guardrail after an accident, and it would be really hard to reconstruct what happened."
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