157484
156228
Skywatching

The second biggest bang

What sort of explosion can blow a hole about 15 times the diameter of the Milky Way in a cluster of galaxies?

That is, a hole 1.5 million light years across.

This is what has happened in a cluster of galaxies in the constellation Ophiuchus. The radiation blast from this event could have wiped out all life in entire galaxies. The only bigger explosion we know of is the Big Bang, the birth of our universe, almost 14 billion years ago.

Ophiuchus, a constellation representing a man fighting a snake (the constellation of Serpens), is high in our skies on summer nights. It actually touches the ecliptic, the path in the sky along which we see the Sun and planets move.

That qualifies it as one of the constellations of the Zodiac, but who wants a Zodiac with 13 signs?

The stars forming the constellation all lie in our immediate neighbourhood in our galaxy, the Milky Way. When we look at that patch of sky and beyond, our telescopes show us a huge cluster of other galaxies, lying about 400 million light years away and containing thousands of members.

We can estimate the amount of energy required to blow that hole; it was about 5e54 Joules, that is, five followed by 54 zeroes.

Using Einstein's equation E equals m c-squared, we find this is equivalent to the total annihilation and conversion into energy of 5.5e34 tonnes of matter. That is, the annihilation of 30 million suns.

The cavity took something like 300 million years to form, so blowing it would require the annihilation of one sun every 10 years.

Considering the Sun is an energy machine that has been keeping us and all the other planets in the Solar System warm for 4.5 billion years, and will continue doing it for about as long again, the object blowing that cavity must be something very special.

The best candidate is a huge black hole. These occur at the centre of galaxies like ours.

To blow that cavity, the black hole needs to have something like 270 million times the mass of the Sun.

Matter being pulled in by the intense gravitational attraction of a black hole forms a spinning disc rather like water flowing down a drain in a bath.

Some of the material goes into the black hole, producing intense bursts of X-rays as it goes, and some of it is ejected as high-speed jets perpendicular to the disc. These jets are moving at almost the speed of light and collide with surrounding material. The X-rays and the jets blow away surrounding dust and gas, causing a cavity.

Blowing that cavity required the black hole to be fed a steady supply of stars and planets. What disrupted a galaxy to the point where lots of stars were sent into the black hole is not clear. It could have been a galactic collision. Our galaxy is due to collide with the Andromeda Galaxy in about 4.5 billion years.

We probably don't have to worry though, because we see other galactic collisions and so far only one galaxy cluster with a hole in it.

The ingredients for life as we know it are available throughout the observable universe. Moreover there are almost certainly many earth-like planets.

There were probably living things in the Ophiuchus Cluster before the hole was blown, but almost as certainly the radiation blast would have wiped them out.

There are many things in the universe over which we have no control whatsoever. However, almost all the stars in the universe get to live out their lives in the standard fashion, along with their planets. We will probably be around until our Sun grows old and becomes a red giant star.

  • The Sun crosses the celestial equator on the 19th, marking the vernal equinox.
  • Venus remains prominent in the west after sunset.
  • Mars, Jupiter and Saturn are clustered low in the southeast before dawn
  • Mercury lies very low in the dawn glow.
  • The moon will be new on the 24th. 


More Skywatching articles

154547
About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

Previous Stories