Interstellar disaster a historic moment for us
LOOKING up to the night sky it can be tempting to think that the universe is a peaceful, largely unchanging place. However, in many cases, that is far from the truth.
One very visible example of that was the brilliant meteor seen over parts of Tasmania and Victoria a few Friday evenings ago — caused by an object that was probably about the size of a soccer ball, or a little larger, burning up in our atmosphere.
On a much larger scale, astronomers can detect far more violent events taking place in our galaxy and others.
This year, they have been delighted to have obtained the best-ever record of a star being literally torn apart by passing too close to a black hole.
We’re often asked about black holes in the Planetarium, and school students are fascinated to learn what would happen to them if they got too close to such an object.
The answer is that it would result in a most unpleasant ending, with the black hole’s gravity on one side of the body being so much greater than the other that one would be stretched out, in a process that has been rather humorously called “spaghettification”.
The event that astronomers have recently witnessed is called a tidal disruption event, or TDE for short. As with the above example, the gravity of the black hole on the side of the star closest to it was so much greater than on the other side that the star was stretched and ripped apart.
The series of records began when the event was discovered using an instrument used in a project called the All-Sky Automated Survey for Supernovae, or ASAS-SN for short. These instruments, operated by the Las Cumbres Observatory global telescope network, regularly observe the whole sky, mainly looking for supernovae (exploding stars) but they find, in addition, some other transient events. The discovery was made using the unit located in South Africa, in January. The event has been given the name ASASSN-19bt.
Astronomers have detected and studied such events before, but only some time after the rise in radiation had begun — meaning that they were missing important early observations.
However, on this occasion another instrument provided plenty of missing data. It was the Transiting Exoplanet Survey Satellite, called TESS. That instrument is primarily intended for the discovery of planets orbiting other stars by watching for the tiny drops in light as the planets pass in front of their “parent” stars, but it can also record changes in light caused by other events.
In the case of ASASSN-19bt, TESS data include measurements of the rise in brightness before it was discovered by the South African instrument. In fact, the TESS information has helped astronomers to pinpoint the time when the rise in brightness started: it began on January 21 (actually, in the small hours of the morning of January 22 Tasmanian time), more than eight days before it was detected from South Africa. Additional observations were made by the space-based Swift and XMM-Newton instruments, observing in ultraviolet, optical and x-ray ranges.
A curious result of careful examination of the data is that after an initial early brightening, it faded temporarily before continuing its rise in brilliance. It’s not certain what caused this, but this observation may be an important clue in working out the detail of exactly what happens during one of these violent, energetic events.
Indeed, the amount of energy released as the star was torn to shreds is mind-boggling. At its peak on March 5 our time, the radiation reached a level about 34 billion times that of our Sun.
Of course, anything orbiting the star, such as any planets it may have had, were also involved in this catastrophe.
However, if you feel a little perturbed by that thought, remember this all happened a long time ago. The galaxy in which this event took place is about 375 million light years away, meaning that the light of the event has taken that long to travel to us. That was a time over 100 million years before dinosaurs appeared on Earth!