The Top Space Questions

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Space is vast, and as far as we know, infinite. Stars, galaxies, planets, and nebulae populate the universe. Space Telescope Science Institute

Astronomy and space exploration are topics that really get people thinking about far away worlds and distant galaxies. Stargazing under a starry sky or surfing the Web looking at images from telescopes always fires up the imagination. Even though a telescope or pair of binoculars, stargazers can get a magnified view of everything from distant worlds to nearby galaxies. And, that act of stargazing spurs a LOT of questions.

Astronomers get asked a lot of those questions, as do planetarium directors, science teachers, scout leaders, astronauts, and many others who research and teach the subjects. Here are some of the most-often-asked questions that astronomers and planetarium people get about space, astronomy, and exploration and collected them along with some pithy answers and links to more detailed articles! 

Where Does Space Begin?

The standard space-travel answer to that question puts the "edge of space" at 100 kilometers above Earth's surface. That boundary is also called the "von Kármán line", named after the Theodore von Kármán, the Hungarian scientist who figured it out.

Earth's atmosphere seen from the ISS
Earth's atmosphere looks very thin when compared to the rest of the planet. The green line is airglow high in the atmosphere, caused by cosmic rays striking the gases up there. This was shot by astronaut Terry Virts from the International Space Station. The legal definition of space is that it begins at the top of the atmosphere. NASA

How Did the Universe Begin?

The universe began some 13.7 billion years ago in an event called the Big Bang. It was not an explosion (as is often depicted in some artwork) but more of a sudden expansion from a tiny pinpoint of matter called a singularity. From that beginning, the universe has expanded and grown more complex.

Big Bang, conceptual image
Most depictions of the beginning of the universe show it almost as an explosion. It was really the beginning of the expansion of space and time, from a small point that contained the entire universe. The first stars formed a few hundred million years after the expansion began. Our universe is now 13.8 billion years old and measures 92 billion light-years across. HENNING DALHOFF / Getty Images

What is the Universe Made of? 

This is one of those questions that has an answer that is quite mind-expanding. Basically, the universe consists of galaxies and the objects they contain: stars, planets, nebulae, black holes and other dense objects. The early universe was largely hydrogen with some helium and lithium, and the first stars formed from that helium. As they evolved and died, they created heavier and heavier elements, which formed second- and third-generation stars and their planets.

The timeline of the universe
This represents a timeline of the universe from the Big Bang through the present. At the left is the "birth event" of the cosmos, known as the "Big Bang". NASA / WMAP Science Team

Will the Universe Ever End?

The universe had a definite beginning, called the Big Bang. It's ending is more like the "long, slow expansion". The truth is, the universe is slowly dying as it expands and grows and gradually cools. It will take billions and billions of years to cool completely and stop its expansion. 

How Many Stars Can We See at Night?

That depends on many factors, including how dark the skies are where. In light-polluted areas, people see only the brightest stars and not the dimmer ones. Out in the countryside, the view is better. Theoretically, with the naked eye and good seeing conditions, an observer can see around 3,000 stars without using a telescope or binoculars. 

What Types of Stars Are Out There?

Astronomers classify stars and assign "types" to them. They do this according to their temperatures and colors, along with some other characteristics. Generally speaking, there are stars like the Sun, which live their lives for billions of years before swelling up and gently dying. Other, more massive stars are called "giants" and are usually red to orange in color. There are also white dwarfs. Our Sun is properly classified as a yellow dwarf. 

hertzsprung-russell diagram
This version of the Hertzprung-Russell diagram plots the temperatures of stars against their luminosities. The position of a star in the diagram provides information about what stage it is in, as well as its mass and brightness. A star's "type" depends on its temperature, age, and other characteristics plotted on diagrams such as this. European Southern Observatory

Why Do Some Stars Appear To Twinkle?

The children's nursery rhyme about "Twinkle, twinkle little star" actually poses a very sophisticated science question about what stars are. The short answer is: the stars themselves don't twinkle. Our planet's atmosphere causes starlight to waver as it passes through and that appears to us as twinkling. 

How Long Does a Star Live?

Compared to humans, stars live incredibly long lives. The shortest-lived ones can shine for tens of millions of years while the old-timers can last for many billions of years. The study of stars' lives and how they are born, live, and die is called "stellar evolution", and involves looking at many types of stars to understand their life cycles. 

The Cat's Eye Nebula
This is what a sun-like star looks like as it dies. It's called a planetary nebula. The Cat's Eye planetary nebula, as seen by Hubble Space Telescope. NASA/ESA/STScI

What is the Moon Made of? 

When the Apollo 11 astronauts landed on the Moon in 1969, they collected many rock and dust samples for study. Planetary scientists already knew the Moon is made of rock, but the analysis of that rock told them about the Moon's history, the composition of the minerals that make up its rocks, and the impacts that created its craters and plains. It's a largely basaltic world, which implies heavy volcanic activity in its past.

What are Moon Phases?

The Moon's shape appears to change throughout the month, and its shapes are called the phases of the Moon. They are a result of our orbit around the Sun combined with the Moon's orbit around Earth. 

Lunar phases
This image shows the phases of the Moon and why they happen. The center ring shows the Moon as it orbits around the Earth, as seen from above the north pole. Sunlight illuminates half the Earth and half the moon at all times. But as the Moon orbits around the Earth, at some points in its orbit the sunlit part of the Moon can be seen from the Earth. At other points, we can only see the parts of the Moon that are in shadow. The outer ring shows what we see on the Earth during each corresponding part of the moon's orbit. NASA

What's in the Space Between Stars?

We often think of space as the absence of matter, but actual space is not really all that empty. The stars and planets are scattered throughout the galaxies, and between them is a vacuum filled with gas and dust. The gases between galaxies are often there due to a galaxy collision that rips gases away from each of the galaxies in involved. In addition, if conditions are right, supernova explosions can also drive hot gases out into intergalactic space.

What's it Like to Live and Work in Space? 

Dozens and dozens of people have done it, and more will in the future! It turns out that, aside from the low gravity, higher radiation hazard, and other dangers of space, it's a lifestyle and a job. 

What Happens to a Human Body in a Vacuum?

Do the movies get it right? Well, not actually. Most of them depict messy, explosive endings, or other dramatic events. The truth is while being in space without a spacesuit WILL kill whoever is unlucky enough to be in that situation (unless the person gets rescued very, very quickly), their body probably won't explode. It's more likely to freeze and suffocate first. Still not a great way to go.

What Happens When Black Holes Collide?

People are fascinated by black holes and their actions in the universe. Until very recently, it's been tough for scientists to measure what happens when black holes collide. Certainly, it's a very energetic event and would give off a lot of radiation. However, another cool thing happens: the collision creates gravitational waves and those can be measured! Those waves are also created when neutron stars collide!

black holes colliding to create gravitational waves
When two supermassive black holes collide and merge, some of the excess energy from the event is broadcast as gravitational waves. These can be detected on Earth using very delicate instruments at the LIGO observatory. The SXS (Simulating eXtreme Spacetimes) Project

There are many more questions that astronomy and space spur in people's minds. The universe is a big place to explore, and as we learn more about it, the questions will continue to flow!

Edited and updated by Carolyn Collins Petersen.