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Why do stars only come out at night? The stars are in the sky both day and night. During the day our star, the Sun, makes our sky so bright that we cannot see the much dimmer stars. At night, when the sky is dark, the light of the stars can be seen.
In this lesson, students use a model to investigate why the stars are visible at night but disappear when the Sun comes out during the day. In the activity, Star Projector, students use paper cups to project stars onto a sky picture, and observe what happens to these stars when a flashlight acts as a model of the Sun. Preview activity.
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Discover why the sky is dark at night and why space appears dark
Learn how infrared energy from stars causes space and the night sky to appear dark.
© MinutePhysics (A Britannica Publishing Partner)
•Discover why the sky is dark at night and why space appears dark
•View an animation to understand the difference between supernovae and neutron stars
•Max Planck and the birth of quantum mechanics
Why is the sky dark at night? You might think the answer is obvious, the sun isn't up. But the only reason the sky looks blue during the day is that sunlight scatters off of the atmosphere. If we didn't have an atmosphere, like on the moon, the sky would always be dark even when the sun is shining. So let's rephrase the question.
Why is space dark? Space is full of stars, countless stars, which are all about as bright as the sun. And in an infinite eternal universe no matter what direction you picked, if you look far enough in that direction, you would see a star or galaxy. So the whole sky should be as bright as the sun, night and day.
And since it's not, does the darkness of the night sky mean that there is some distance away from us when stars and galaxies just stop? A boundary between something and nothing? An edge to the universe?
Not exactly. All of our evidence seems to indicate that space has no edge, but the universe itself does, not a spatial edge but a temporal one. As far as we know, the universe had a beginning, or at least a time about 13.7 billion years ago when the universe was so small and crumpled up with itself that our standard notion of space and time breaks down. And since only a finite amount of time has passed since this so-called beginning, that means that some of the stars necessary to fill up the brightness in every direction are so far away that light from them plain hasn't had time to reach us yet.
It's as if the universe were a big thunderstorm and we're still waiting to hear the thunder from the really distant stars. But wait, it's better than that. Since light takes time to travel across the universe, when we point our telescopes at something really far away, we're actually seeing that part of the universe as it was when the light was emitted.
So when we look at 13.5-billion-year-old light, it's not that we don't see stars just because the light from them hasn't gotten to us yet, we don't see any stars because we're getting a peek at the universe before any stars had formed, a starless universe. Now, that sounds to me like a pretty good reason why we look up and see a dark night sky, but it's not.
- 4 min
A star is a huge sphere of very hot, glowing gas. Stars produce their own light and energy by a process called nuclear fusion. Fusion happens when lighter elements are forced to become heavier elements. When this happens, a tremendous amount of energy is created causing the star to heat up and shine. Stars come in a variety of sizes and colors.
On an average clear night, you can literally see thousands of stars in the sky. When morning comes, all the stars have disappeared. Well, its not exactly the case as one star can still be seen in the sky, that is the Sun. The stars that we would otherwise see are blocked by the light from the Sun and ...
12 apr 2024 · Star, any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources. This article describes the properties and evolution of individual stars. Included in the discussion are the sizes, energetics, temperatures, masses, and chemical compositions of stars.
Death. At the beginning of the end of a star’s life, its core runs out of hydrogen to convert into helium. The energy produced by fusion creates pressure inside the star that balances gravity’s tendency to pull matter together, so the core starts to collapse.
