Compact objects

DTU Space builds and operates space-based x-ray and gamma-ray telescopes for astronomical satellites that study black holes and neutron stars, so-called 'compact objects'.

Black holes and neutron stars are the remains of very massive stars that ended their lives in supernova explosions. They are called compact objects because of their extremely high densities. For example, a neutron star has a radius of about 10 kilometres but a mass somewhat larger than that of the Sun. Consequently the density of a neutron star is about 100,000 tons per teaspoonful. The density of a black hole is even greater.

Compact objects have extremely strong gravitational fields. The gravity on the surface of a neutron star is, for example, hundreds of billions of times greater than that on the surface of the Earth. Near a black hole the gravitational field is so strong that even light cannot escape its pull, which is why black holes themselves do not emit radiation (light). However, black holes so strongly affect their surroundings that the region around the hole can be detected by the x-ray and gamma-ray radiation produced there.

For black holes to be detected by x-rays and gamma-rays there needs to be a companion star because the high-energy radiation we see is produced when a compact object attracts matter from the other, normal star. As the gas is pulled from the star and approaches the black hole, or neutron star, it forms a huge rotating disk around the object, called an accretion disk. Matter in the disk is heated up to several million degrees because of the gravitational energy released by the falling matter thanks to the huge gravitational field of the compact object. Therefore the disk glows with the high energy radiation that can be detected by x-ray and gamma-ray telescopes.

Some neutron stars also emit x-rays and gamma-rays from their surface. Two examples are pulsars and magnetars which are relatively young neutron stars with amazingly strong magnetic fields. In the latter case, the magnetic field is so strong that it causes 'star quakes' and the gamma-rays it emits are powerful enough to ionise atoms in the Earth's atmosphere. For other, older neutron stars, high-energy radiation is produced when matter from the accretion disk falls onto the neutron star, forming a layer on its surface. Thanks to the very high temperature and density of this layer, the accreted matter is compressed to such an extent that thermonuclear reactions explode across the neutron star's surface, producing observable x-ray bursts lasting from a few seconds to a few minutes.

Using x-ray and gamma-ray observations of these extremely energetic phenomena, on and around black holes and neutron stars, it is possible to study the laws of physics under the most extreme conditions, which can never be reproduced in laboratories on Earth.

DTU Space has built and operates two x-ray telescopes called JEM-X (The Joint European Monitor for X-rays) onboard ESA's space observatory, INTEGRAL, which has observed the sky in x-rays and gamma-rays since 2002. DTU Space is also currently involved in an important contribution to NASA's gamma-ray space telescope, NuStar, which were launched in 2012, carrying with it two hi-tech mirrors with very special coating provided by DTU Space. 

Facts about compact objects

'Compact Objects' is an umbrella name for some of the universe's most exotic and mysterious objects, such as white dwarves, black holes and neutron stars. They are all the final stage in the life of a star, with the type of compact object determined solely by the original mass of the star.