Andromeda and Milky Way Might Collide Sooner Than We Think

The merger of the Milky Way and Andromeda galaxy won’t happen for another 4 billion years, but the recent discovery of a massive halo of hot gas around Andromeda may mean our galaxies are already touching. University of Notre Dame astrophysicist Nicholas Lehner led a team of scientists using the Hubble Space Telescope to identify an enormous halo of hot, ionized gas at least 2 million light years in diameter surrounding the galaxy.

The Andromeda Galaxy is the largest member of a ragtag collection of some 54 galaxies, including the Milky Way, called the Local Group. With a trillion stars — twice as many as the Milky Way — it shines 25% brighter and can easily be seen with the naked eye from suburban and rural skies.

Think about this for a moment. If the halo extends at least a million light years in our direction, our two galaxies are MUCH closer to touching that previously thought. Granted, we’re only talking halo interactions at first, but the two may be mingling molecules even now if our galaxy is similarly cocooned.

Lehner describes halos as the “gaseous atmospheres of galaxies”.  Despite its enormous size, Andromeda’s nimbus is virtually invisible. To find and study the halo, the team sought out quasars, distant star-like objects that radiate tremendous amounts of energy as matter funnels into the super-massive black holes in their cores. The brightest quasar, 3C273 in Virgo, can be seen in a 6-inch telescope! Their brilliant, pinpoint nature make them perfect probes.



“This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger”

“As the light from the quasars travels toward Hubble, the halo’s gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range,” said J. Christopher Howk , associate professor of physics at Notre Dame and co-investigator. “By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.”

Astronomers have observed halos around 44 other galaxies but never one as massive as Andromeda where so many quasars are available to clearly define its extent. The previous 44 were all extremely distant galaxies, with only a single quasar or data point to determine halo size and structure.

Andromeda’s close and huge with lots of quasars peppering its periphery. The team drew from about five years’ worth of observations of archived Hubble data to find many of the 18 objects needed for a good sample.

The halo is estimated to contain half the mass of the stars in the Andromeda galaxy itself, in the form of a hot, diffuse gas. Simulations suggest that it formed at the same time as the rest of the galaxy. Although mostly composed of ionized hydrogen — naked protons and electrons —  Andromeda’s aura is also rich in heavier elements, probably supplied by supernovae. They erupt within the visible galaxy and violently blow good stuff like iron, silicon, oxygen and other familiar elements far into space. Over Andromeda’s lifetime, nearly half of all the heavy elements made by its stars have been expelled far beyond the galaxy’s 200,000-light-year-diameter stellar disk.

You might wonder if galactic halos might account for some or much of the still-mysterious dark matter. Probably not. While dark matter still makes up the bulk of the solid material in the universe, astronomers have been trying to account for the lack of visible matter in galaxies as well. Halos now seem a likely contributor.

The next clear night you look up to spy Andromeda, know this: It’s closer than you think!


Raj Ondhia



Hubble Catches a Stellar Exodus in Action

Using NASA’s Hubble Space Telescope, astronomers have captured for the first time snapshots of fledgling white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs.

White dwarfs are the burned-out relics of stars that rapidly lose mass, cool down, and shut off their nuclear furnaces. As these glowing carcasses age and shed weight, their orbits begin to expand outward from the star cluster’s packed downtown. This migration is caused by a gravitational tussle among stars inside the cluster. Globular star clusters sort out stars according to their mass, governed by a gravitational billiard-ball game where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster’s core, while lighter stars pick up speed and move across the cluster to the edge. This process is known as “mass segregation.” Until these Hubble observations, astronomers had never definitively seen the dynamical conveyor belt in action.


Astronomers used Hubble to watch the white-dwarf exodus in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in our Milky Way galaxy. The cluster resides 16,700 light-years away in the southern constellation Tucana.

“We’ve seen the final picture before: white dwarfs that have already sorted themselves out and are orbiting in a location outside the core that is appropriate for their mass,” explained Jeremy Heyl of the University of British Columbia (UBC), Vancouver, Canada, first author on the science paper. The team’s results appeared in the May 1 issue of The Astrophysical Journal.

“But in this study, which comprises about a quarter of all the young white dwarfs in the cluster, we’re actually catching the stars in the process of moving outward and segregating themselves according to mass,” Heyl said. “The entire process doesn’t take very long, only a few hundreds of millions of years, out of the 10-billion-year age of the cluster, for the white dwarfs to reach their new home in the outer suburbs.”

“This result hasn’t been seen before, and it challenges some ideas about some of the details of how and when a star loses its mass near the end of its life,” added team member Harvey Richer of UBC.

Using the ultraviolet-light capabilities of Hubble’s sharp-eyed Wide Field Camera 3, the astronomers examined 3,000 white dwarfs, tracing two populations with diverse ages and orbits. One grouping was 6 million years old and had just begun their journey. Another was around 100 million years old and had already arrived at its new homestead far away from the center, roughly 1.5 light-years, or nearly 9 trillion miles, away.

Only Hubble can detect these stars because ultraviolet light is blocked by Earth’s atmosphere and therefore doesn’t reach ground-based telescopes. The astronomers estimated the white dwarfs’ ages by analyzing their colors, which gives them the stars’ temperatures. The hottest dwarfs shine fiercely in ultraviolet light.

The dwarfs were tossed out of the rough-and-tumble cluster center due to gravitational interactions with heftier stars orbiting the region. Stars in globular clusters sort themselves out by weight, with the heavier stars sinking to the middle. Before flaming out as white dwarfs, the migrating stars were among the most massive in the cluster, weighing roughly as much as our Sun. The more massive stars burned out long ago.

The migrating white dwarfs, however, are not in a hurry to leave. Their orbits expand outward at about 30 miles an hour, roughly the average speed of a car traveling in the city. The dead stars will continue this pace for about 40 million years, until they reach a location that is more appropriate for their mass.

Although the astronomers were not surprised to see the migration, they were puzzled to find that the youngest white dwarfs were just embarking on their journey. This discovery may be evidence that the stars shed much of their mass at a later stage in their lives than once thought.

About 100 million years before stars evolve into white dwarfs, they swell up and become red giant stars. Many astronomers thought that stars lose most of their mass during this phase by blowing it off into space. But the Hubble observations reveal that the stars actually dump 40 percent to 50 percent of their bulk just 10 million years before completely burning out as white dwarfs.

“This late start is evidence that these white dwarfs are losing a large amount of mass just before they become white dwarfs and not during the earlier red giant phase, as most astronomers had thought,” said Richer. “That’s why we are seeing stars still in the process of moving slowly away from the center of the cluster. It’s only after they lose their mass that they get gravitationally pushed out of the core. If the stars had shed most of their weight earlier in their lives, we wouldn’t see such a dramatic effect between the youngest white dwarfs and the older ones that are 100 million years old.”

Although the white dwarfs have exhausted the hydrogen fuel that makes them shine as stars, these stellar relics are among the brightest stars in this primordial cluster because their brilliant hot cores have been exposed, which are luminous largely in ultraviolet light. “When a white dwarf forms, they’ve got all this stored-up heat in their cores, and the reason we can see a white dwarf is because over time they radiate their stored thermal energy slowly into space,” Richer explained. “They’re getting cooler and less luminous as time goes on because they have no nuclear sources of energy.”


Sources: NASA, ESA, Hubblesite