10-03-18 Quasars（类星体） are active and very powerful black holes at the centre of distant galaxies. The black holes are extremely massive weighing between 100 million and 10 billion solar masses and rotating around the super massive black hole is a disc o...

09-10-30 Many of us have been rescued from unfamiliar territory by directions from a Global Positioning System (GPS) navigator（导航器）. GPS satellites send signals to a receiver in your GPS navigator, which calculates your position based on the location of...