There is a supermassive black hole at the center of our galaxy, and it's not alone. There is also likely a forest of binary black holes, neutron stars, and white dwarfs. All of these emit gravitational waves as they gradually spiral ever closer together. These gravitational waves are too faint for us to detect at the moment, but future observatories will be able to observe them. This poses an interesting astronomical challenge.
Arabic
French
Spanish
Portuguese
Deutsch
Turkish
Dutch
Italiano
Russian
Romaian
Portuguese (Brazil)
Greek