Science-based Q&A

Browse the questions and answers below to deepen your knowledge of astronomy as a science. Learn the facts about the universe’s fascinating array of celestial bodies, from planets to stars to black holes. Discover the difference (and evidence for) dark energy and dark matter. And learn about worlds closer to home such as Venus and Saturn.

The Q&As presented here probe a variety of topics. When will Earth see its last total solar eclipse? How many stars are in the Milky Way galaxy? How do you calculate the density of a black hole? Read on to discover the answers, and ask your own astronomy questions by sending a note to

Closed universe (top), open universe (middle), and flat universe (bottom). 

Is space flat or curved?

Locally, spacetime is curved by the presence of massive objects. The total mass and energy density of the universe also has an effect on the overall curvature of space.

The dark matter concentrations in the galaxy cluster Abell 1689 are tinted blue. Astronomers determined the location of those concentrations from gravitational lensing.
NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI)

What is dark matter?

Dark matter is a mysterious type of matter that doesn't interact with any form of electromagnetic radiation, i.e., light. Although we’ve never detected dark matter directly, a large amount of evidence points to its existence.

Artist's rendering of a supermassive black hole.

How big is a black hole?

Black holes are singularities: points of infinitely small volume with infinite density. However, the amount of a mass concentrated in a black hole varies, and the mass determines how wide the black hole's sphere of influence is.

Two-dimensional representation of the three-dimensional curvature of spacetime around a massive object. 

Are black holes real? If so, who discovered them?

The concept of a black hole was first contrived in by John Michell 1783. For a long time, many notable scientists, including Albert Einstein, believed black holes were merely theoretical. However, in the last century, astronomers have gathered a good deal of observational evidence for the existence of black holes.

Map of the eras of the Big Bang. 

How did the universe begin? What happened during the Big Bang?

The Big Bang marked the beginning of the universe's expansion from a singularity — a single point that was infinitely small, infinitely hot, and infinitely dense. Cosmologists have designated several distinct eras for the universe's evolution from the first moments after the Big Bang to a billion years later.

Our expanding universe

What is dark energy?

Cosmologists have invoked the concept of dark energy to explain the accelerated expansion of the universe, but the nature of dark energy remains one of the most pressing questions facing modern cosmology.

Smattering of distant galaxies imaged in the Ultraviolet Coverage of the Hubble Ultra Deep Field Project.
NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

Is there a center of the universe?

The universe began as a singularity that started expanding in the Big Bang. But the Big Bang was no regular explosion. Rather, space itself expanded, so there is no center of the entire universe. The observable universe, on the other hand, is a different story.

Smattering of distant galaxies imaged in the Ultraviolet Coverage of the Hubble Ultra Deep Field Project.
NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

How many galaxies are there in the universe?

Astronomers determine the number of galaxies in the universe by counting up the number visible in a tiny portion of the sky, and then accounting for all the regions of the observable universe. A 2013 study estimates that there are 225 billion galaxies in the observable universe.

Planck temperature map of universe

What is the age of the universe?

Determining the age of the universe requires a knowledge of the universe's expansion rate, as well as its density and composition. Cosmologists currently set the age of the universe at about 13.77 billion years.

The Big Dipper asterism in the constellation Ursa Major. 
Starry Night Software / A.Fazekas

What are constellations?

In 1930, the International Astronomical Union (IAU) divided the sky into 88 constellations. Each constellation is defined by an imaginary boundary on the sky and named after a classical star pattern within those boundaries. So when we say a star is “in” a particular constellation, we mean it lies within the IAU-defined boundaries of...

Chandra X-ray photograph of young supernova remnant Cassiopeia A.
NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.

How long do stars live? How do stars die?

Both the life and death of a star depend on its mass. Generally speaking, the more massive a star, the faster it burns its fuel and the shorter its life. The most massive stars meet their end in a supernova explosion after only a few million years of fusion, while the tiniest stars continue...

Image credit: NASA

What is a star, exactly?

A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.

Image credit: Hubble

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.

Comet ISON (C/2012 S1)

What is a comet? Where do comets come from?

Like asteroids, comets are suspected to be remnants of planet formation in the solar system about 4.6 billion years ago. But while asteroids are generally comprised of rock and metal, comets are more akin dirty snowballs. Comets primarily originate in the Kuiper Belt and the Oort Cloud.