A new study using data from NASA’s Hubble Space Telescope finds that star formation in the nearby Andromeda galaxy has seen a 500-million-year decline, with an even steeper decline over the past 40 million years. Andromeda, a spiral galaxy comparable in size to our Milky Way, is close enough to be seen with the naked eye from areas with dark skies. Located about 2.5 million light years from Earth, practically our cosmic backyard, Andromeda offers astronomers the opportunity to examine its stellar populations in detail, which will lead to a better understanding of the past of galaxies like ours.

The results were published Monday in The Astrophysical Journal.

To reach this conclusion, the researchers combined data from two Hubble surveys: the Hubble Andromeda Panchromatic Treasury and the Hubble South Andromeda Panchromatic Treasury. Together, these two studies mapped two-thirds of Andromeda’s disk in great detail. In total, the team measured about 200 million individual stars across the galaxy, giving them a detailed picture of Andromeda’s past activity.

“We need to measure individual stars because they are the fossil record of galaxy formation. Hubble is the only telescope that can deliver high enough spatial resolution over a large enough area to be able to do so in Andromeda,” said Ben Williams, an astronomer at the University of Washington and co-author of the study.

Massive stars are bluer and shorter-lived, while less massive stars are redder and longer-lived. As a result, areas that have experienced recent star formation tend to have a higher fraction of blue stars, while areas with less recent star formation tend to have a redder population. The team divided the Andromeda images into thousands of squares, spanning 300 light years on each side of the square, and determined the history of star formation within each plot to obtain a complete view of the galaxy’s past.

Previous research showed that the Andromeda galaxy experienced a dramatic burst of star formation about 2 billion years ago, likely due to a past interaction or merger with another galaxy. Since then, star formation has been steadily declining.

Astronomers measure the rate of star formation in terms of mass, or the amount of gas and dust, converted into stars per year. The researchers calculated that, 500 million years ago, Andromeda formed stars at a rate of about one solar mass per year. However, the rate of formation dropped by about half 40 million years ago. The current rate has plummeted further, to about one-fifth of our Sun’s mass per year.

The team also examined whether that decline was constant across the galaxy or concentrated in certain areas. They found that much of the recent star formation occurred in a star-forming ring located about 32,000 light years from the center of the galaxy. As a result, much of the decline they measure is due to decreased activity within that ring.

Rather than being the result of the reduction of material from which new stars can form, the decline is more likely a natural decline from its previous, more active state.

“It’s like after running a marathon, sometimes you have to take a breather,” said Tobin Wainer, lead author at the University of Washington.

The team also investigated whether there was any connection between the decline in activity in Andromeda and its proximity to the satellite galaxy M32 (Messier 32). The galaxy M32 is separated from Andromeda by about 16,000 light years in the plane of the sky; however, its 3D location in space is uncertain. As a result, astronomers are unsure if and when it may have interacted with Andromeda in the past.

“One of the main motivations for this program was to investigate possible interactions between M32 and the Andromeda disk,” said Zhuo Chen, a co-author from the University of Washington.

Data from Hubble Andromeda’s Panchromatic South Treasure survey allowed the team to study the history of star formation in Andromeda near M32. They found that this area showed signs of less star formation compared to other regions. The timing of this decline, which this study shows began about 60 million years ago, could help constrain the timing of when the M32 galaxy interacted with the Andromeda disk.

“We can’t explicitly say that we’re seeing a decrease in star formation due to M32. But it’s there, and it’s definitely the most likely suspect,” Wainer said.

The team plans to continue analyzing Hubble data and combine it with data from ground-based observatories to gain additional insights into Andromeda’s history.

“This archival data has great scientific value. Andromeda is important because it is a close enough neighbor that we can see it in great detail and at the same time get a global perspective,” said co-author Raja GuhaThakurta from the University of California, Santa Cruz.

An even greater global perspective is likely to come from NASA’s Nancy Grace Roman Space Telescope after its launch on Sunday, August 30. Roman’s gigantic field of view can cover at least 100 times more area than Hubble at near-infrared wavelengths in a single observation. A recently approved Roman observing program will image the entire Andromeda disk and areas of its surrounding halo, allowing astronomers to measure hundreds of millions of stars and enabling groundbreaking new science.

The Hubble Space Telescope has been operating for more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.