The best meteor shower of the year is on now! The Perseids will be at their best in the early hours of Saturday and Sunday, in the morning after midnight. The Moon will rise after midnight, but it should not be too bright, allowing patient sky gazers to see 30 or so meteors an hour. These shooting stars appear to radiate from the constellation Perseus in the northern sky. Perseus is a hero of Greek legend, known for slaying the Medusa. To the ancient Greeks the head of the Medusa was known as a separate constellation – the Gorgon. But now the Medusa is seen as the stars making a head hanging from the left hand of Perseus. The bright variable star Algol is the eye of the Medusa- its name comes from the Arabic “Ra’s al-Ghul” or the Demon’s Head. This star is an eclipsing binary and gets significantly dimmer over the course of 10 hours every three days as the dimmer of the pair crosses our line of sight to the brighter star. If you want to check the brightness of this star while waiting for some meteors, it is due to be at a minimum on August 7, 10, 13 and 16, but unfortunately not always during our night-time.
If meteors and stars are too close to home for you, the constellation is home to a huge cluster of galaxies about 240 million light years away. Known as the Perseus Cluster, it is part of an even larger grouping – the Perseus-Pisces Supercluster. Superclusters are part of the large scale structure of the universe. Back in 1977, based on a study of this supercluster, it was suggested that galaxies are not randomly distributed throughout the Universe. Instead they are found in clusters, which in turn are found in superclusters. Disks of superclusters intersect to form walls around voids, in which very few galaxies are found. Voids typically have diameters in the hundreds of million light years range. The void next to this supercluster is the Taurus void – right next door in the sky!
Credit: X-ray: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA/ESA/Hubble Heritage (STScI/AURA) & Univ. of Cambridge/IoA/A. Fabian
The best studied member of the Perseus cluster is the bright elliptical galaxy NGC 1275. It is a strong source of radio waves and X-rays. In 2003 ripples and cavities in the gas in the middle of the cluster were detected with the Chandra X-ray Observatory. The ripples seem to be caused by explosions around a supermassive black hole in the central galaxy. This spreads out as sound waves, travelling across hundreds of thousands of light years and keeping the cluster gases hot. This might explain why trillions of stars haven’t formed around this galaxy. Given that it has a lot of gas that is emitting a lot of energy, it really should have cooled down by now and settled into making stars. The sound wave corresponds to a B flat, only 57 octaves below middle C. The tone has remained constant for about two and a half billion years!
Listen live to “What is the Stars?” on Mondays and Fridays on RTÉ’s lyricfm at 22:45.
