Secrets of Ancient Stars of the Milky Way’s Halo

NASA Milky Way Halo

 

“Finding old stars could also lead to the discovery of new planets. Maybe we can find some ancient civilizations around these old stars,” suggested astrophysicist, Wei-Chun Jao, research scientist at Georgia State University about the oldest stars developed in the early stage of the Milky Way’s formation, making them about eight to twelve billion years old. They’re found in the halo, a roughly spherical component of the galaxy that formed first, in which old stars move in orbits that are highly elongated and tilted, hinting at their age through their motion, rotation rate, and chemistry.

 Cool Subdwarfs–Earliest Generations of Stars n our Milky Way

Just like humans, stars have a lifespan: birth, youth, adulthood, senior and death. This 2017 study focused on old or “senior citizen” stars, also known as cool subdwarfs, that are much older and cooler in temperature than the sun. 

Maybe we can find some ancient civilizations around these old stars”

Cool subdwarfs are among the earliest generations of stars that formed in our Milky Way, and hence are metal poor. Subdwarfs have less than a tenth – and in some rare cases only a millionth – the abundances of heavy elements like iron compared to our Sun. Although astronomers have demonstrated that gas giant planets favor metal-rich hosts, they are still studying how metallicity affects the occurrence rates of Earth-like planets. Some studies even suggest that Earth-like planets in the habitable zone prefer stellar hosts with slightly sub-solar metallicities. 

In the study, published in The Astronomical Journal, astronomers conducted a census of our solar neighborhood to identify how many young, adult and old stars are present. They targeted stars out to a distance of 200 light years, which is relatively nearby considering the galaxy is more than 100,000 light years across. A light year is how far light can travel in one year. This is farther than the traditional horizon for the region of space that is referred to as “the solar neighborhood,” which is about 80 light years in radius.

The Milky Way –Our Galaxy Formed Around Oldest Surviving Stars in the Universe

“The reason my horizon is more distant is that there are not a lot of senior citizens (old stars) in our solar neighborhood,” said Jao. “There are plenty of adult stars in our solar neighborhood, but there’s not a lot of senior citizens, so we have to reach farther away in the galaxy to find them.”

Not a lot of senior citizens in our solar neighborhood”

The astronomers first observed the stars over many years with the 0.9 meter telescope at the United State’s Cerro Tololo Inter-American Observatory in the foothills of the Chilean Andes. They used a technique called astrometry to measure the stars’ positions and were able to determine the stars’ motions across the sky, their distances and whether or not each star had a hidden companion orbiting it.

Discovered Two Rare Binary Stars

The team’s work increased the known population of old stars in our solar neighborhood by 25 percent. Among the new subdwarfs, the researchers discovered two old binary stars, even though older stars are typically found to be alone, rather than in pairs.

Milky Way’s Ancient K Stars – The Best Bet for Planets With Life?

“I identified two new possible double stars, called binaries,” Jao said. “It’s rare for senior citizens to have companions. Old folks tend to live by themselves. I then used NASA’s Hubble Space Telescope to detect both stars in one of the binaries and measured the separation between them, which will allow us to measure their masses.”

Jao also outlined two methods to identify these rare old stars. One method uses stars’ locations on a fundamental map of stellar astronomy known as the Hertzsprung-Russell (H-R) diagram. This is a classic technique that places the old stars below the sequence of dwarf stars such as the Sun on the H-R diagram, hence the name “subdwarfs.”

Fast Facts About Old Stars of the Universe

Older Stars Move Faster

The authors then took a careful look at one particular characteristic of known subdwarf stars — how fast they move across the sky. “Every star moves across the sky,” Jao said. “They don’t stay still. They move in three dimensions, with a few stars moving directly toward or away from us, but most moving tangentially across the sky. In my research,” Jao said, “I’ve found that if a star has a tangential velocity faster than 200 kilometers per second, it has to be old. So, based on their movements in our galaxy, I can evaluate whether a star is an old subdwarf or not. In general, the older a star is, the faster it moves.”

In 2018, results from the European Space Agency’ Gaia mission, which is measuring accurate positions and distances for millions of stars in the Milky Way, will make finding older stars much easier for astronomers. Determining the distance of stars is now very labor intensive and requires a lot of telescope time and patience. Because the Gaia mission will provide a much larger sample size, Jao says the limited sample of subdwarfs will grow, and the rarest of these rare stars — binary subdwarfs — will be revealed.

Strange, Ancient Stars of the Milky Way -Challenge Current Understanding

Gaia Space Telescope’s Data for 1.3 billion Stars

The European Space Agency released its second set of Gaia data and with it came new information about 1.7 billion stars in the Milky Way. That included 1.3 billion stars’ apparent motion in the sky and actual velocities for over 7 million stars. The researchers behind the newest study used this information to study the galaxy in “phase space”—how its stars’ velocities vary with their position.

The stellar disk of the Milky Way is a dynamically active place, where spiral arms and the Galactic bar leave their marks on the orbits of stars like ripples in a pond. Stars in the disk not only orbit around the Milky Way, but gyrate up and down out of the midplane of the disk as they circle the galactic center. Meanwhile, cool subdwarfs in the halo have more random motions, orbiting the galactic center across the full range of inclinations and eccentricities. 

This second Gaia data release takes the census of our Galaxy to an entirely new level, as it includes three-dimensional positions and two-dimensional motions of more than 1.3 billion stars, along with their distances, brightnesses and colors that can help paint a bigger picture about the star as its color gives information on its surface temperature; the hotter the star, the more blue it is; the cooler the star, the more red it is.

Gaia has achieved this impressive feat of data collection by sweeping across the sky while taking observations in a large circular motion. “In about one minute, Gaia measures about a hundred thousand stars on the sky,” said Anthony Brown. It takes around two months for Gaia to look at the whole of the sky and in this data release most stars will have been recorded on average about 70 times.

Animations built from Gaia data allows the positions of stars to be seen in 3D, allowing scientists to rotate around a particular star cluster for example to see it from all sides.

Over its lifetime of 5 years, the Gaia satellite will take 29 independent measurements of the whole sky, giving astronomers an unprecedented look at how the stars in our Milky Way behave and evolve; data gleaned from Gaia will help to literally reconstruct the history of the Milky Way.

The Last Word -Potential for Life

In an email to The Daily Galaxy, Wei-Chun Jao wrote: “As for the potential life around these low metallicity stars, these subdwarfs were once young, and they follow the same stellar evolutionary tracks as their metal rich counterparts. The host star’s activity is one of the key elements to trigger life. These nearby subdwarfs are inactive now, but this doesn’t mean they were quiet billions years ago. If a planet was formed around a subdwarf, this planet could be bombarded with activities in the early stages of stellar evolution. If there were life formed on that planet, did it imply that planet could have much stable environment to develop life? The life there may not have a wide diversity because of lacking host star’s activities to trigger genetic mutation, but it should have a relative quiet and stable environment to sustain a few life forms.

“After all,” Jao expalins, “as long as a planet is formed under the right and sustainable environments, life could be developed. The extra dosage of stellar activities from metal rich stars could possibly contribute to develop a wide variety of life forms, instead of just a few.”

Image credit top of page: NASA

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Wei Chun Jao and  Georgia State University

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