Article by: Holly Preece, PhD student at the Armagh Observatory and Planetarium
NASA’s Hubble space telescope was launched on 24th April 1990. It was the first optical space telescope to be launched into orbit and has been one of the most productive scientific instruments ever built. It orbits the Earth every 95 minutes and has almost completed its 28th orbit of the Sun. So far in 2018 it has released a series of beautiful high resolution images and aided a number of scientific investigations which shall be reviewed.
Milky Way Bulge
The bulge of the Milky Way is a big, dense region of stars at the centre of our galaxy. Our Solar system is 26,000 light years away from the bulge in the disk of the Milky Way. The bright blue stars in the image are young hot stars in the disk between us and the bulge. The redder stars are the older more evolved giant stars with slow, scattered motions. The smaller white stars are younger and are more rapidly orbiting the galactic centre.
Hubble Parameter Measurements with NGC1015 and NGC3972
One of the HST’s recent scientific projects was to make distance measurements to 19 galaxies to help improve the precision of the measured Hubble parameter. Edwin Hubble, namesake of both the HST and the Hubble parameter, was the first astronomer to observe galaxies other than our own. He used these measurements to provide evidence that the universe is expanding and that everything is moving away from everything else. The Hubble parameter is a measure of this expansion. It is defined as the ratio of the distance to the object and the velocity which object is moving away with. The value of the parameter has varied substantially since Hubble’s first measurement due to improvements in measurement techniques.
The distances to galaxies can be measured by observing Cepheid variables or Type 1a Supernovae. Cepheid variables are a type of star which cyclically and periodically change in brightness. The time period over which the brightness changes over is related to its luminosity. Type 1a supernovae are explosions of white dwarf stars. All 1a supernovae have roughly the same luminosity. If the absolute magnitude of the star is known the distance to it can be calculated. Galaxies with both observable Cepheids and Type 1a Supernovae are best to make precise distance measurements.
NGC 1015 is spiral galaxy located in the whale constellation Cetus.
NGC 3972 is spiral galaxy in Ursa Major which is the constellation most famous for housing the dig dipper.
‘Red and Dead’ NGC1277
NGC 1277 is an old elliptical galaxy which has had no star formation whatsoever in the last 10 billion years. Galaxies like this are often named red and dead and provide a good snap shot into the conditions of the early universe. Studies suggest this galaxy initially had intense burst of star formation, much higher than that of our own galaxy. It is mainly populated with metal rich stars. No galaxies or globular clusters are close enough for it to merge with and fuel further star formation.
Ghost Galaxy NGC1052-DF2
NGC 1052-DF2 is an ultra diffuse ‘ghost’ galaxy. It is almost entirely see through and the galaxies behind it are clearly visible. It is so difficult to see that no one realized it was a galaxy for quite a long time after it had been observed. It is associated with NGC 1052 which is an elliptical galaxy in Cetus. It has star forming regions and young clusters so is still an active galaxy.
It is particularly peculiar in that it only has 1/400th of the regular amount of dark matter and 1//200 the amount of stars typically observed in galaxies. Dark matter is an essential building block of the universe and is an essential component for many galaxy formation theories. The lack of observed dark matter raises many questions, particularly regarding the galaxies formation. Interestingly it also suggests dark matter isn’t just an artifact of behavior of regular matter under gravity and that galaxies can exist without it.