Typical signatures of active galaxies are variable brightness, broad emission lines, and strong radio emission. The period since the Earth's formation from planetesimals, measured to be 4.6 billion years. Estimated to be roughly 12-16 billion years. Most are less than 100 million years. Ages from 1 million to a few billion years have been reported.
Age is difficult to measure for individual stars, but possible to measure for clusters of stars.
In the telescopic image of a star, a small disk caused by optical effects. The nearest spiral galaxy comparable to our own, about 670 kpc away. An eclipse in which the light source is almost, but not quite, covered, leaving a thin ring of light at mid-eclipse. Antimatter happens to be rare in our universe. Matter and antimatter annihilate on contact to produce gamma rays.