April 9, 2013
NGC 1637, a barred spiral galaxy in Eridanus
Image Credit: ESO
NGC 1637 is a barred spiral galaxy that lies about 38 million light-years away in the southern constellation of Eridanus (the River) while it is receding from us at approximately 717 kilometers per second.
The galaxy is lit by the glow of about fifty billion stars slowly evolving over millions and billions of years. Its spiral structure shows up in this image as a very distinct pattern of bluish clusters of young stars, glowing gas clouds and obscuring dust lanes. However, it is rather asymmetric—lopsided—with the arm to the upper left a lot longer than its counterpart on the other side, probably due to gravitational interactions with other galaxies as they pass close by.
In its center lurks a supermassive black hole, while x-rays reveal bright sources that appear to be a neutron star and, in one of the outer spiral arms of the galaxy, a stellar-mass black hole that formed relatively recently (in the last million years or so) when a massive star exhausted its nuclear fuel, exploded as a supernova and left behind a black hole which is now pulling in gas from a companion star.
In 1999 scientists discovered a Type II supernova (SN 1999em) in this galaxy and followed its slow fading over the following years.
A supernova occurs when a star explodes in the final phase of its life. A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.
The exploding star can become billions of times as bright as the Sun before gradually fading from view. At its maximum brightness, the exploded star may outshine an entire galaxy. Because of its relative proximity, SN 1999em got bright enough to be seen even with small telescopes, and it became one of the best observed supernovae of its day.
Supernova explosions are enriching the intergalactic gas with elements like oxygen, iron, and silicon that will be incorporated into new generations of stars and planets. When the star explodes, a lot of energy is generated, and those materials are scattered at very high speed. Not only that, new elements like nickel are created in the blast. It turns out the total nickel in SN 1999em was about 0.02 times the mass of the Sun, roughly a thousand times the volume of the Earth.
This composite image was taken with ESO’s Very Large Telescope at the Paranal Observatory in Chile, using three different color filters.