Key Takeaways:
- Using data from the 2006 Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS), NASA's Hubble Space Telescope detected between 18 and 37 blue straggler stars within the Milky Way's central bulge, the first such detection in this region.
- Blue stragglers, stars appearing younger than their surrounding stellar population, are hypothesized to form from binary star systems where mass transfer rejuvenates the less massive star.
- The discovery supports the theory that star formation in the Milky Way's bulge ceased billions of years ago, as the observed number of blue stragglers is significantly lower than would be expected if young stars were still forming.
- Hubble's high resolution and stability were crucial in distinguishing the motion of the blue stragglers from foreground stars, confirming their location within the galactic bulge.
Blue stragglers are so named because they seemingly lag behind in the aging process, appearing younger than the population from which they formed. While they have been detected in many distant star clusters, and among nearby stars, the blue stragglers have never been seen inside the core of our galaxy.
It is not clear how blue stragglers form. A common theory is that they emerge from binary pairs. As the more massive star evolves and expands, the smaller star gains material from its companion. This stirs up hydrogen fuel and causes the growing star to undergo nuclear fusion at a faster rate. It burns hotter and bluer, like a massive young star.
The findings support the idea that the Milky Way’s central bulge stopped making stars billions of years ago. It now is home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there have long since exploded as supernovae.
“Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood,” said Clarkson from Indiana University in Bloomington. “Many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge.”
The discovery followed a 7-day survey in 2006 called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Hubble peered at 180,000 stars in the crowded central bulge of our galaxy, 26,000 light-years away. The survey was intended to find hot Jupiter-class planets that orbit close to their stars. In doing so, the SWEEPS team also uncovered 42 oddball blue stars with brightness and temperatures typical for stars much younger than ordinary bulge stars.
The observations clearly indicate that if there is a young star population in the bulge, it is very small. It was not detected in the SWEEPS program. Blue stragglers long have been suspected to be living in the bulge, but had not been observed because younger stars in the disk of our galaxy lie along the line of sight to the core, confusing and contaminating the view.
Astronomers used Hubble to distinguish the motion of the core population from foreground stars in the Milky Way. Bulge stars orbit the galactic center at a different speed than foreground stars.
Plotting their motion required returning to the SWEEPS target region with Hubble 2 years after the first observations were made. The blue stragglers were identified as moving along with the other stars in the bulge.
“The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm’s length, and within this region, Hubble sees about a quarter-million stars toward the bulge,”Clarkson said. “Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field.”
From the 42 candidate blue stragglers, the investigators estimate 18 to 37 are likely genuine. The remainder could be a mix of foreground objects and, at most, a small population of genuinely young bulge stars.
“The SWEEPS program was designed to detect transiting planets through small light variations,” said Kailash Sahu from the Space Telescope Science Institute in Baltimore, Maryland. “Therefore, the program could easily detect the variability of binary pairs, which was crucial in confirming these are indeed blue stragglers.”
Blue stragglers are so named because they seemingly lag behind in the aging process, appearing younger than the population from which they formed. While they have been detected in many distant star clusters, and among nearby stars, the blue stragglers have never been seen inside the core of our galaxy.
It is not clear how blue stragglers form. A common theory is that they emerge from binary pairs. As the more massive star evolves and expands, the smaller star gains material from its companion. This stirs up hydrogen fuel and causes the growing star to undergo nuclear fusion at a faster rate. It burns hotter and bluer, like a massive young star.
The findings support the idea that the Milky Way’s central bulge stopped making stars billions of years ago. It now is home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there have long since exploded as supernovae.
“Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood,” said Clarkson from Indiana University in Bloomington. “Many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge.”
The discovery followed a 7-day survey in 2006 called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Hubble peered at 180,000 stars in the crowded central bulge of our galaxy, 26,000 light-years away. The survey was intended to find hot Jupiter-class planets that orbit close to their stars. In doing so, the SWEEPS team also uncovered 42 oddball blue stars with brightness and temperatures typical for stars much younger than ordinary bulge stars.
The observations clearly indicate that if there is a young star population in the bulge, it is very small. It was not detected in the SWEEPS program. Blue stragglers long have been suspected to be living in the bulge, but had not been observed because younger stars in the disk of our galaxy lie along the line of sight to the core, confusing and contaminating the view.
Astronomers used Hubble to distinguish the motion of the core population from foreground stars in the Milky Way. Bulge stars orbit the galactic center at a different speed than foreground stars.
Plotting their motion required returning to the SWEEPS target region with Hubble 2 years after the first observations were made. The blue stragglers were identified as moving along with the other stars in the bulge.
“The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm’s length, and within this region, Hubble sees about a quarter-million stars toward the bulge,”Clarkson said. “Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field.”
From the 42 candidate blue stragglers, the investigators estimate 18 to 37 are likely genuine. The remainder could be a mix of foreground objects and, at most, a small population of genuinely young bulge stars.
“The SWEEPS program was designed to detect transiting planets through small light variations,” said Kailash Sahu from the Space Telescope Science Institute in Baltimore, Maryland. “Therefore, the program could easily detect the variability of binary pairs, which was crucial in confirming these are indeed blue stragglers.”
