The universe is a wild and weird place, and two recent findings from orbiting space telescopes illustrate just how bewildering it can be: a monster-sized black hole in one of the tiniest galaxies known and a huge exoplanet that is making its star appear deceptively older than it is.
First that black hole. The findings come from the Hubble Space Telescope, announced yesterday by NASA. Many, if not most, galaxies are thought to have huge black holes at their centers, including our own Milky Way. But this one is unusual because the host galaxy is so small.
The galaxy, M60-UCD1, is a dwarf galaxy which contains 140 million stars packed into a diameter of about 300 light-years; that’s very small for a galaxy and is only 1/500th the diameter of the Milky Way. If you were able to transport yourself to inside this galaxy, the night sky would be brimming with over 1 million stars. Compare that to a night sky on Earth, where only about 4,000 stars are typically visible.
The black hole residing in its centre, though, is five times the mass of the one in the centre of our galaxy (and that one has a mass of four million Suns!). How did this come about? The discovery means that big black holes in smaller galaxies may be fairly common, and that dwarf galaxies may be what is left over of much larger galaxies which were stripped of most of their stars during collisions with other galaxies. It is estimated that M60-UCD1 may have once contained 10 billion stars, but then all of the stars and dark matter in the outer part of the galaxy were lost after after it passed close to the centre of a neighbouring galaxy, M60. The two galaxies may eventually fully merge.
“We don’t know of any other way you could make a black hole so big in an object this small,” said University of Utah astronomer Anil Seth, lead author of an international study of the dwarf galaxy published in Thursday’s issue of the journal Nature.
The black hole in M60-UCD1 makes up about 15 percent of the galaxy’s mass, compared to less than 0.01 percent of the Milky Way’s mass that its black hole constitutes. As Seth noted, “That is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1.”
Now on to that exoplanet that makes its star think that it is older than it actually is. The planet, WASP-18b, is a giant “hot Jupiter” which orbits very close to its star, WASP-18, about 330 light-years from Earth. WASP-18b is about 10 times as massive as Jupiter and orbits its star in only 23 hours. As observed by the Chandra X-Ray Observatory, it is orbiting so close that it seems to be having a dampening effect on the star’s magnetic field.
Astronomers had estimated that WASP-18 is between 500 million and 2 billion years old, which is still young for a star. It was expected that the star would still be giving off a lot of X-rays due to its young age, since stars gradually lose their X-ray and magnetic activity as they age. But unexpectedly, it was found that WASP-18 isn’t giving off any X-rays at all. How could this be? It may be that the star’s magnetic field is being disrupted by the massive planet close by. The theory is that the gravity of the planet is weakening convection inside the star, the process where hot gases move around inside the star. Having a smaller convection zone than most stars, WASP-18 is more vulnerable to the gravitational tidal forces.
There is also a higher-than-usual amount of lithium in WASP-18, which is abundant in younger stars. As a star ages, however, the lithium is carried into the star’s interior by the same convection forces, where it is destroyed. But if there is less convection, then more of the lithium should survive.
The results were published in the July 2014 issue of Astronomy and Astrophysics.
These new findings emphasize how appearances can be deceiving in a universe which itself is stranger than we can imagine.