Astronomy Questions & Answers

What’s a black hole? What’s inside a black hole? What’s the difference between a reflecting and refracting telescope? This is the place to come for answers to almost all the astronomy questions you can think of — and many you haven’t. Find answers to your equipment and hobby questions, as well as astronomy facts about the other planets in the solar system, stars, and galaxies.

Is Andromeda Galaxy headed for a clash with the Milky Way? What’s the faintest thing you can see with a ground-based telescope? From stargazing advice to stellar science, browse this Astronomy Q&A category, browse astronomy questions by topic, or email us with your own question at to deepen your knowledge of astronomy as a hobby and a science.

Why Do Stars Twinkle?

Though it wouldn’t work so well in the nursery rhyme, a star’s twinkling actually has a technical term, astronomical scintillation: the effect of our planet’s atmosphere on starlight.

Why do comets have tails?

Comets develop tails as they approach perihelion—the place in their orbits when they are closest to the sun. The sun’s heat causes some of the material in a comet to vaporize, which in turn releases dust particles that were trapped in the ice.

How big is the Sun?

The Sun is more than 330,000 times as massive than the Earth. It has a diameter of nearly 1.4 million kilometers (865,000 miles), and its volume could enclose about 1.3 million Earths.

What is a meteor shower?

Meteor showers occur when Earth passes through a stream of meteoric material. The brief streaks of luminescence we call meteors are caused by meteoroids burning up as they pass through the atmosphere.

Astrology vs Astronomy: What’s the Difference?

The practices of astrology and astronomy have common roots, but they have evolved into two separate fields. Astronomy studies positions, motions, and properties of celestial objects. Astrology attempts to study how those positions, motions, and properties affect people and events on Earth.

What’s Up at Star Parties?

When amateur astronomers and their families plan a vacation, they are increasingly likely to plan it around a star party. The big amateur gatherings have been growing in recent years, and new ones devoted especially to dark-sky observing are sprouting all over. True deep-sky star parties — as opposed to the more traditional, general-interest conventions…

How can a telescope have an f/ratio of f/42?

I was amazed at Jim Melka’s beautiful picture of Mars on page 136 of the January 2006 issue but puzzled by the caption, which said that he used a 12-inch reflector at f/42. How is this possible? Knowing that a telescope’s f/ratio is its focal length divided by its aperture, you’re probably imagining poor Jim…

The 12 Planets of Our Solar System

Will all eight planets ever line up on the same side of the Sun?

Will there ever be a moment when all eight major planets are in a straight line on the same side of the Sun? Jean Meeus addresses this in Mathematical Astronomy Morsels (Willmann-Bell, 1997). He points out that you have to start by defining the question precisely. Let’s reduce the problem to two dimensions and ask…

Andromeda Galaxy

Is it true that Andromeda Galaxy is moving toward us?

Is it true that the Andromeda Galaxy is blueshifted and moving toward us? How can this happen in an expanding universe? The Andromeda Galaxy (M31) is indeed approaching us, by about 300 kilometers (190 miles) per second measured with respect to the Sun. If you subtract the Sun’s orbital motion around our galaxy (about 230…