This illustration shows the Milky Way galaxy's inner and outer halos. A halo is a spherical cloud of stars surrounding a galaxy. Astronomers have proposed that the Milky Way's halo is composed of two populations of stars. The age of the stars in the inner halo, according to measurements by the Paranal Observatory, is 11.5 billion years old. The measurements suggest the inner-halo stars are younger than the outer-halo population, some of which could be 13.5 billion years old. Credit: NASA, ESA, and A. Feild (STScI)

Jason Kalirai of the Space Telescope Science Institute and The Johns Hopkins University’s Center for Astrophysical Sciences, in Baltimore, Maryland has asked the next-to-impossible… a lady’s age. In this case, the lady is a white dwarf star.

The story she tells is crucial to understand the history of the Milky Way. At home in the inner regions of the galactic halo, this diminutive stellar spinster reveals how our galaxy formed over billions of years from smaller galaxies. In Kalirai’s study, he discovered white dwarf stars are eleven and a half billion years old – an age younger than the Milky Way’s first generation stars. The original population of stars began lighting up our neighborhood around two billion years after the Big Bang. According to previous research, these stars were estimated to be about ten billion to fourteen billion years old. However, their ages were bases on analysis done on normal stars in the inner halo. This new study helps to aid the concept that our galaxy’s halo consists of many layers which formed periodically over billions of years.

“One of the biggest questions in astronomy is, when did the different parts of the Milky Way form?” Kalirai said. “Sun-like stars live for billions of years and are bright, so they are excellent tracers, offering clues to how our galaxy evolved over time. However, the biggest hindrance we have in inferring galactic formation processes in the Milky Way is our inability to measure accurate ages of Sun-like stars. In this study, I chose a different path: I studied stars at the end of their lives to determine their masses and then connected those masses to the ages of their progenitors. Given the nature of these dead stars, their masses are easier to measure than Sun-like stars.”

Why choose white dwarf stars? Kalirai based his research on them because they are surmised to be among the very first to form in the halo… almost as old as the Universe itself. They are similar to tracing a tree’s growth by its ring system – providing vital clues about how the Milky Way was born and how it evolved. “The Milky Way’s halo represents the premier hunting ground in which to unravel the archaeology of when and how the galaxy’s assembly processes occurred,” Kalirai explained.

White dwarf stars have remarkable properties, yet they are very simple. These stripped cores of normal hydrogen-burning stars are about 1 million times denser than matter on Earth. This means that a tablespoon of material from a white dwarf's surface would weigh as much as a school bus on Earth. White dwarfs also have no fuel to generate energy, and most of their atmospheres contain a single atom, hydrogen. Credit: NASA, ESA, and A. Feild and J. Kalirai (STScI)

Much like a lady’s mannerisms, white dwarf stars reveal their properties through their distinct spectrum. According to the news release, Kalirai analyzed their colorful signatures using archival spectroscopic data from the European Southern Observatory’s Very Large Telescope at the Paranal Observatory in Chile. The spectroscopic information is part of the SN Ia Progenitor Survey (SPY), a census of white dwarf stars in the Milky Way.

Spectroscopic signature provides information about a star’s temperature and mass, allowing Kalirai to delve deeper into the galaxy’s inner halo structure. “The hottest white dwarfs are the descendants of Sun-like stars that have just extinguished their hydrogen fuel,” he explained. “The masses of these white dwarfs are proportional to the masses of their progenitors, and we can use that mass to establish the age of the parent stars.”

So how did he determine this well cloaked halo age? By comparing the mass of the halo stars with six newly formed white dwarfs located in evolved globular cluster – M4. This well-studied spherical grouping of stars has been a preferred subject of many Hubble Space Telescope studies and astronomers are confident in their assessments that it formed about twelve and half billion years ago. Through Hubble’s Advanced Camera Survey imaging, Kalirai located almost two thousand white dwarf stars hidden in the archives.

From there, he used the same method as the one employed on the halo white dwarfs and combined it with spectroscopic data collected by the W.M. Keck Observatory in Hawaii. The results were that halo white dwarf stars were heavier than the ones located in M4. This finding shows the progenitor stars are equally heavy and younger than M4’s members. How did he know they were younger? The more massive stars deplete their hydrogen quickly and extinguish themselves faster than the light-weights.

Even if Kalirai’s findings are based on a small sample, it still shows the halo stars could be two different populations. Through his research, he shows the evolution of the Milky Way started with old globular clusters and dwarf galaxies. These combined a few hundred million years after the Big Bang, forming our galaxy’s halo. Over the next few billion years this became the framework and stars began forming in the inner halo. During the process the Milky Way consumed older dwarf galaxies and the ancient stars took up residence in the outer halo.

“In the previous work, the inner population was shown to be different from the outer population in terms of the velocities and chemical abundances of the stars,” Kalirai said. “There were no constraints, however, on whether there was an age difference between the two populations. Now, our work suggests an age for the inner halo stars.

“We know some of the remote globular clusters in the outer halo are much older than the inner halo stars, perhaps around 13.5 billion years old,” Kalirai continued. “So, our prediction is that if you find white dwarfs in the outer halo, they would have formed from older generations of Sun-like stars. The present day masses of stars in the generation that are now forming white dwarfs would be lower, and therefore the white dwarf masses – which we can measure – will also be lower.”

Kalirai is looking to the future where he’s anxious to test his new methods on more halo white dwarfs… a method which may ultimately help further our understanding of our galaxy’s formation history.

“One of the interesting questions about the inner halo stars is, did all of them form at the same time, or did they form over a span of time?” Kalirai said. “A sample of 20 to 30 white dwarfs would allow us to see if the inferred ages from the white dwarf masses span from 11 billion to 13 billion years. That could tell us that the accretion events that helped build up the Milky Way kept happening for several billion years, as opposed to all predominantly happening at one epoch.”

Original News Source: Hubble Site News Release. For Further Reading: Kalirai’s “Nature” Paper.

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