What makes stars appear brighter in the sky

In a later chapter, we will see that if we can measure how much energy a star emits and we also know its mass, then we can calculate how long it can continue to shine before it exhausts its nuclear energy and begins to die. Astronomers are careful to distinguish between the luminosity of the star the total energy output and the amount of energy that happens to reach our eyes or a telescope on Earth.

Stars are democratic in how they produce radiation; they emit the same amount of energy in every direction in space. Consequently, only a minuscule fraction of the energy given off by a star actually reaches an observer on Earth. If you look at the night sky, you see a wide range of apparent brightnesses among the stars. Most stars, in fact, are so dim that you need a telescope to detect them. If all stars were the same luminosity—if they were like standard bulbs with the same light output—we could use the difference in their apparent brightnesses to tell us something we very much want to know: how far away they are.

Imagine you are in a big concert hall or ballroom that is dark except for a few dozen watt bulbs placed in fixtures around the walls. Since they are all watt bulbs, their luminosity energy output is the same. But from where you are standing in one corner, they do not have the same apparent brightness. Those close to you appear brighter more of their light reaches your eye , whereas those far away appear dimmer their light has spread out more before reaching you.

In this way, you can tell which bulbs are closest to you. In the same way, if all the stars had the same luminosity, we could immediately infer that the brightest-appearing stars were close by and the dimmest-appearing ones were far away.

To pin down this idea more precisely, recall from the Radiation and Spectra chapter that we know exactly how light fades with increasing distance. The energy we receive is inversely proportional to the square of the distance.

If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. If it is three times farther away, it will look nine three squared times dimmer, and so forth. Alas, the stars do not all have the same luminosity. Actually, we are pretty glad about that because having many different types of stars makes the universe a much more interesting place.

But this means that if a star looks dim in the sky, we cannot tell whether it appears dim because it has a low luminosity but is relatively nearby, or because it has a high luminosity but is very far away. To measure the luminosities of stars, we must first compensate for the dimming effects of distance on light, and to do that, we must know how far away they are. Distance is among the most difficult of all astronomical measurements. We will return to how it is determined after we have learned more about the stars.

For now, we will describe how astronomers specify the apparent brightness of stars. Around B. There he prepared a catalog of nearly stars that included not only their positions but also estimates of their apparent brightnesses. Its name comes from the Greek word for "scorching. It's actually a double star system, with a very bright primary and a dimmer secondary star. Sirius is visible from late August in the early mornings until mid-to-late March and lies 8.

Astronomers classify it as a type A1Vm star, based on their method of classifying stars by their temperatures and other characteristics. Canopus was well known to the ancients and is named either for an ancient city in northern Egypt or the helmsman for Menelaus, a mythological king of Sparta.

It's the second brightest star in the night sky, and mainly visible from the Southern Hemisphere. Observers who live in the southern regions of the Northern Hemisphere can also see it low in their skies during certain parts of the year.

Canopus lies 74 light-years away from us and forms part of the constellation Carina. Astronomers classify it as a type F star, which means it's slightly hotter and more massive than the Sun. It's also a more aged star than our Sun. Rigel Kentaurus, also known as Alpha Centauri, is the third brightest star in the night sky. It's one of the most famous stars in the sky, and first-time travelers to the Southern Hemisphere are often eager to view it. Rigel Kentaurus is not just one star.

It's actually part of a three-star system, with each star looping around with the others in an intricate dance.

It lies 4. It may be about the same age as our Sun and is in roughly the same evolutionary period in its life. Stargazers often learn it as they star-hop from the stars of the Big Dipper to find other stars in the sky.

There's an easy way to remember it: simply use the curve of the Big Dipper's handle to "arc to Arcturus. Astronomers classify it as a type K5 star which, among other things, means it is slightly cooler and a bit older than the Sun.

Vega is the fifth-brightest star in the night sky. Its name means "the swooping eagle" in Arabic. Vega is about 25 light-years from Earth and is a Type A star, meaning it is hotter and somewhat younger than the Sun. Astronomers have found a disk of material around it, which could possibly hold planets.

Stargazers know Vega as part of the constellation Lyra, the Harp. It's also a point in an asterism star pattern called the Summer Triangle , which rides through the Northern Hemisphere skies from early summer to late autumn. The sixth brightest star in the sky is Capella. Its name means "little she-goat" in Latin, and it was charted by many ancient cultures, including the Greeks, Egyptians, and others.

Capella is a yellow giant star, like our own Sun, but much larger. Astronomers classify it as a type G5 and know that it lies some 41 light-years away from the Sun. Capella is the brightest star in the constellation Auriga, and is one of the five bright stars in an asterism called the "Winter Hexagon". Rigel is an interesting star that has a slightly dimmer companion star that can be easily seen through telescopes.

Rigel's name comes from the Arabic word for "foot" and it is indeed one of the feet of the constellation Orion, the Hunter.

Astronomers classify Rigel as a Type B8 and have discovered it is part of a four-star system. It, too, is part of the Winter Hexagon and is visible from October through March each year. Traveling Exhibits. Community Programs. Power of Children Awards. Visiting Artist Application. Corporate Donations. Planned Gifts. Renew your Donor Membership. Current Members. Renew Your Membership. Member FAQ. The Children's Museum Today's Hours: 10 am—5 pm. Sports Legends Experience Indoor: 10 am—5 pm Outdoor: 10 am—5 pm Our outdoor experiences are weather dependent.

Buy Tickets. Things to Do. Museum Blog. Two Main Factors There are actually two factors that influence how bright a star appears to our eyes: 1 The Star's Actual Brightness Some stars are naturally more luminous than others, so the brightness level from one star to the next can be significantly different.

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