In astronomy, extragalactic refers to objects beyond our own Milky Way galaxy. Most commonly, it is used to describe galaxies other than our own. The term originates from the Greek words meaning “outside of” and “Milky Way Galaxy”.
Extragalactic objects can be studied in a variety of ways. One way is to use powerful telescopes to observe their visible light emissions. This method of study is limited by the fact that only a small fraction of the universe is emitting visible light at any given time. Additionally, extragalactic objects are often so far away that their light has been stretched into invisible wavelengths (redshift) by the time it reaches us. For these reasons, other methods must be employed to learn about extragalactic objects.
One common method for studying extragalactic objects is through radio astronomy. Radio waves are not affected by redshift in the same way as visible light, so they can provide information about very distant objects. Additionally, because radio emission is produced by different mechanisms than optical emission (such as rotating neutron stars), radio telescopes can detect different types of astronomical phenomena than optical telescopes can.
X-ray astronomy also provides valuable information about extragalactic objects. X-rays are emitted by hot gas near accreting black holes and binary star systems with high mass ratios; they are also scattered off of clusters of galaxies and absorbed by material along the line-of-sight between an observer and a distant object. Because X-rays have short wavelengths, they too are subject to redshift; however, this effect can be used to measure the amount of matter along the line-of sight between an observer and a distant object (known as “gravitational lensing”). By combining data from multiple wavelength bands (optical through X-ray), astronomers can piece together a more complete picture of extragalactic objects