The universe is composed primarily of galaxies — egg- and spiral-shaped collections of hundreds of billions of stars — at mind-melting distances from Earth.
Our Milky Way is a member of a small cluster of galaxies called the Local Group, a collection of 20 or so galaxies. It and the Andromeda Galaxy, another galaxy in the Local Group, dominate the cluster.
Our Local Group is a lump of galaxies at the edge of a much larger group called the Virgo Supercluster, containing tens of thousands of galaxies.
M81 is the dominant member of another lump near the edge of the Virgo Supercluster. Astronomers call the collection of 34 galaxies, unsurprisingly, the M81 Group.
The M81 Group lies only 12 million light-years from the Milky Way, making it a celestial neighbor.
Sometimes, the members of a galactic cluster are close enough together that they fit into the same telescope field. Such is the case with M81 and M82, two galaxies in the constellation Ursa Major, the Big Bear.
Their telescopic proximity is mirrored in physical fact. The two galaxies are a scant 150,000 light-years from each other. The distance of our Milky Way galaxy to the Andromeda Galaxy, our nearest celestial neighbor, is 2.5 million light-years. (For the record, one light-year is equivalent to about six trillion miles.)
M81 is about 90,000 light-years wide and contains considerably more stars than M82, a galactic lightweight, at only 37,000 light-years wide.
Their relative proximity and M81’s larger mass make M81 gravitationally dominant over M82. That sounds bad for M82, I suppose, but a few million years in its past, things were a lot worse, as we shall see.
First, a bit of background is necessary. Our own Milky Way galaxy is typical of the spiral class of galaxies. When viewed from the top, it looks much like a child’s pinwheel — with curved arms of stars radiating from a central bulge, dense with stars.
We see M81 from the top. In long-exposure images it is one of the best and most beautiful examples of a spiral galaxy seen “face-on,” as amateur telescopists say.
When we look at a spiral galaxy from the side, it resembles a cosmic toothpick with a lump in the center. A dark lane of dust and gas cuts almost all across most of its length.
Some galaxies don’t quite fit the pattern. The Starburst Galaxy, M82 in the Messier catalog, is a particularly nasty case in point. We see it edge-on, but its dust lanes have been disturbed.
In other words, M82 is one messed-up galaxy. Instead of a dark lane along the length of the cigar, you might see one, perhaps two, dark streaks that cut across the galaxy’s width and not its length. What gives?
M82’s problems began with its closeness to M81. Within the last 200 million years, the two galaxies passed very close to each other one or more times. Their gravitational interaction pulled dust and gas out of M82’s spiral arms into its central core, which accounts for the dark dust lanes that cut through M82 vertically.
As a result of that interaction, M82 is going through an intense period of star formation, producing hundreds of millions, perhaps billions, of stars simultaneously. Most of this prodigious star formation happened in M82’s dense central core, where its close encounter with M81 funneled most of the galaxy’s gas and dust.
The burst of star formation seems to have happened in several stages. The first stage began at the initial close encounter of the two galaxies. It lasted for 50 million years, producing stars at the steady rate of 10 solar masses per year. The second phase happened only two or so million years ago. It yielded massive, close-together clusters of stars, many of which are composed of more than 100,000 solar masses.
However, “100,000 solar masses” does not necessarily mean 100,000 sun-like stars. Such “starburst galaxies” tend to produce plenty of stars more massive than our sun.
If they have more than eight or nine times the sun’s stellar material, they burn their hydrogen fuel so rapidly that they live and die in millions of years instead of the usual 10 billion.
When such a massive star dies, it ejects its outer shell of gases in an enormous release of energy called a supernova. Its inner core implodes from millions of miles wide to a space smaller than Earth’s moon.
When so much stellar material collapses to such a tiny volume, the highly-dense dead star generates so much gravity that even light cannot shine from its surface. Those dead stars are called black holes simply because they radiate no light, and therefore, we cannot see them.
Luckily for astronomers, black holes are often surrounded by gas and other materials too far from the star to fall into it during its initial collapse. Over time, some of that material falls rapidly toward the black hole.
As the molecules of gas and dust spiral in, they smash into each other, producing prodigious quantities of X-rays, which are traveling rapidly enough to escape the gravity of the black hole. Thus, we can’t see black holes, but we can detect their X-ray signatures.
Until recently, astronomers had detected two types of black holes. A “stellar” black hole results from a single collapsed star and might be as massive as a few suns clumped together.
“Supermassive” black holes often exist at the centers of galaxies. At the hub of the galactic wheel, millions of solar masses of gas and dust collapsed into a ball of material only a few miles wide.
M81, the face-on spiral that was relatively undisturbed by its nearest approach to M82, has a central, supermassive black hole composed of material that weighs in at 70 million solar masses. By contrast, the Milky Way galaxy has a central black hole formed out of only 4.3 million suns.
In 1999, Andrew Ptak and Richard Griffiths at Carnegie Mellon University used the space-based Advanced Satellite for Cosmology and Astrophysics (ASCA) X-ray telescope to examine M82. They discovered what appears to be a new kind of black hole in the process. This “middleweight” black hole has enough stellar material to form hundreds of stars the size of our sun.
A subsequent investigation by the orbiting Chandra X-ray telescope indicated that the black hole consisted of only 460 solar masses. That’s large compared to a single-star black hole’s mass of 9-25 suns, but it’s downright puny compared to the supermassive black holes at the center of most galaxies.
In M82, millions of stellar black holes have formed in the last 200 million years. Some of them were close enough together to collapse into at least one middleweight black hole, a process that continues to this day, perhaps.
The merger of even two black holes is among the most cataclysmic events the universe has to offer. It produces an enormous deadly (to us, at least) burst of gamma rays that makes a single supernova explosion look like a pop gun by comparison.
The massive burst of star formation and the presence of a middleweight black hole have not been kind to the structure of M82. The galaxy is being torn apart by the normal processes of star birth and star death gone totally out of control.
If all of this seems academic, consider the fate of your own galaxy, the Milky Way. In two billion years or so, the Andromeda Galaxy and the Milky Way will pass close to each other. Their mutual gravitational pull will trigger a new round of star formation in both galaxies.
Soon after, the two galaxies will pass through each other a couple of times, compressing their clouds of hydrogen and triggering yet another round of star birth.
Eventually, the two galaxies will merge into an egg-shaped elliptical galaxy and remain that way until their last stars finally use up their hydrogen fuel and wink out.
Not so with M81 and M82. In the long run, the galaxies will continue their cosmic dalliance, hand in hand but never merging — gravitationally dating but never married.
