Betelgeuse is the primary star in the constellation Orion marking the upper right vertex of the Winter Triangle. It is a variable star changing its apparent brightness over time. With a radius of nearly 1200 times that of our sun, Betelgeuse would extend up to Jupiter if placed into the center of our solar system. This is the first image of a star surface. Image credit ESO.

Vega is the primary star in the constellation Lyra. It rotates at fast speed in that its shape is oval due to strong centrifugal forces. Vega is the second brightest star in the northern hemisphere and lies at a distance of 25.3 light-years in the solar neighborhood. Vega is surrounded by a disk of dust. Image credit NASA/JPL.

The Alpha Centauri Star System

Artist's concept of planet "ALF Cen B b"

In October 2012, astronomers from ESO announced discovery of an Earth-sized planet orbiting the secondary star of the Alpha Centauri triple star system. The size of this planet is the only characteristic which is similar to Earth. In every other respect, the planet is hostile to life. The planet orbits Alpha Centauri B in 3.236 days on a distance of 6 million kilometers, in that the surface temperature is believed to exceed 1000°C. The surface is scorched on one side and freezing on the other as it is most probably tidally locked to its host star.

Anyway, this exoplanet is the closest to Earth. The Alpha Centauri system lies in a distance of 4.366 light-years, which, on astronomical scale, is a close neighbor in the constellation Centaurus visible from the southern hemisphere.

The Sun as seen from Alpha Centauri. Tap to view Sirius and Procyon.

The Sky from Alpha Centauri

With Sky Atlas you can simulate the sky as seen from Alpha Centauri. Select "Atlas Objects" and pick "Named Stars". Tap on Toliman at the top of the list. Then press [View From]. The default view is centered on the Sun and can be shifted to the star's coordinates. The view shows the Sun in Alpha Centauri's antipodal point, which lies in Cassiopeia where it shines as the constellation's brightest star at a magnitude of 0.48 near ε and δ Cassiopaiae (Ruchbah). The W shape of Cassiopeia appears extended by another line from ε Cas to the Sun, forming a zig-zag wave. Sirius is situated next to Betelgeuse in Orion. Like in our sky, Sirius is the brightest star over Alpha Centauri's sky, though a little fainter. Except for noticable positional changes of nearby stars, such as Sirius and Procyon, the constellations look much alike with only minor shifts of stars, say, 100 light-years around -- like Caph and Schedar in Cassiopeia.

Triple Star System

Star sizes and view at the sky centered on Taurus

Alpha Centauri appears like a single star to the unaided eye. It is the fourth brightest star in the sky. The star system consists of two sun-like stars, named Toliman A and Toliman B, and a small, faint red-dwarf star named 'Proxima' (Latin for 'nearest'), because it is currently the nearest star to Earth (4.24ly). It's true space motion at 21.7km/s towards Earth will bring it to a distance of 3.11 light-years in 26,700 years and then recede.

Toliman A is 23% larger, while Toliman B is 14% smaller than our Sun. Proxima measures a mere 1/8th that of the sun an lies 0.237 light-years away from AB. It is still uncertain whether Proxima is gravitationally bound to AB, or on an hyperbolic orbit. If bound, and in that distance, the orbital period would amont to some 500,000 years.

Toliman B orbits the primary star once in 79.9 years, currently heading towards is periapis (closest orbit distance to Toliman A) which it will reach in 2035. Toliman A rotates around its axis in 22 days, 3 days faster than the Sun, while Toliman B is estimated to rotate in 47 days. As seen from Earth, the orbit of B around A is inclined by 79° with an eccentricity of 0.52 which brings the two stars as close as 11.2AU (roughly the Sun - Saturn distance, and as far as 35.6AU (roughly the Sun - Pluto distance).

The system has a high proper motion that will take it next to Beta Centauri in about 4000 years as observed from Earth's vantage. Beta Centauri, 'Hadar', is a different star, some 500 light-years further away.

Possibility of More Planets

Hypothetical planet for Toliman A (tap to enlarge)

Theoretically, Alpha Centauri could harbour more planets, either orbiting close to A or B or in wide distance around AB. In the latter case, planets could not sustain life as we know it because they would be too far away from the stars, thus without a constant source of heat. Nevertheless, the Kepler space telescope has found circumbinary planets, such as Kepler-16b. In order to sustain liquid water, a planet around A needs to orbit in 1.25AU, about halfway between Earth and Mars. Since B is cooler and less luminous, a planet here would need to orbit in 0.7AU, comparable to the distance of Venus. Although a planet around Proxima is not likely, its warm zone is around 0.04AU, resulting in a period of circa 8 days. In such proximity, a planet would be exposed to extensive radiation and flare outbursts, eliminating any hope for existance of life.

System Overview

Binary Orbit

The Stars -- "in English"

On a clear night away from air and light pollution, we can see over 6000 stars with the naked eye while virtually travelling through the past.

Stars lie far away from us out there in space at distances that can no longer be represented in conventional units of distance, such as miles. For this reason, we measure celestial distances in years that visible light requires to travel from a star or other objects - namely in "light-years". The closest star to our sun shines 4.4 light-years away in the constellation of Centaurus. The light of this star (commonly named Rigil Kent or Toliman) that we are seeing at present travelled 4.4 years. In other words, we are seeing the star as it was 4.4 years ago, thus looking into the past.

"Betelgeuse", the primary star of the popular constellation of Orion, is over 420 light-years away, in that we are looking at light that is 420 years old. It began its journey through space roughly in the year 1590 or 20 years before Galileo Galilei discovered the major moons of Jupiter with his self-made telescope.

The asterism, we call the Summer Triangle is composed of the stars Vega (Lyra), Altair (Aquila) and Deneb (Cygnus). Vega is 25.3 light-years away, while Altair is nearer by at 16.8 light-years. The interstellar distance between the two is 14.8 light-years. If observed from Altair, Vega shines as bright as Sirius, the brightest star in our sky or approximately five times brighter than observed on Earth.

As a fun fact, the star of your birth is a star that is as far away in light-years as your age in years. Say, you are now 45 years old, then the star "Errai" in the constellation of Cepheus is currently a candidate for your birth star because you see its light as sent out 45 years ago - when you were born. In the next year, perhaps Rasalhague in Ophiuchus?

Bright stars are not necessarily closer than faint stars. Our sun is a relatively small star while the majority of stars is larger, therefore emitting more light which appears bright even in long distances. For example, the star "Deneb" in Cygnus is over 3,000 light-years away, yet one of the brightest visible stars. In contrast, "Barnard's Star" is a mere 6 light-years away, but much smaller than our sun, so faint that it can only be seen in larger telescopes.

Astronomers often calculate the absolute brightness of a star. While the star light that hits our eyes is apparent brightness, absolute brightness is the light intensity of a star if placed in a distance of 32.6 light-years. Betelgeuse, in that distance, would outshine the brightest phase of Venus.

Stars emit light in various visible colors, such as blue, white, yellow orange and red. The physical explanation is far more complex, however, in simple terms, theses colors are a measure of star temperature, blue being the hottest and youngest stars and red the coolest and oldest stars. Compare it with candle light. The blue light in the bottom center of the flame is hottest, the outer red region is the coolest, though still too hot to touch. Aged stars, like Betelgeuse and Antares in Scorpio (600 light-years), are of striking orange-red color indicating a temperature of ca. 3000 degrees. For comparison, our sun is 5880 degrees hot. In astronomical terms, both "red stars" are near their end of life. Returning to the constellation of Lyra, white Vega exhibits a surface temperature of ca. 9600 degrees and is 455 million years young - astronomically speaking.

Well, there is much more worth knowing about stars; sizes, masses, densities, compositions. Are there planets around other stars? The internet has it all, but for a best start I recommend to watch the starry sky with your bare eyes or a pair of binoculars and muse about the yet unknown.

Measuring Distances to Nearby Stars

Hold your finger about 8 inches (20cm) in front of your eyes. Then close the right eye and remember the position of your finger. Next, open your right eye and close your left. The position of your finger shifted versus the background. The amount of shift is trigonometrically related to the distance of your finger from your eyes. The distance between your eyes is too narrow a baseline for measuring astronomical distances. The widest baseline we can use is the diameter of Earth's orbit around the sun, namely 186.4 million miles (300 million km).

If we measure the angle to a star on two lines of sight, say, once in January and another time in July we obtain a subtle apparent positional change of the star against background stars. The difference of the two angles is a direct measure for distance. The farther away the smaller the angle.

1 AU is the mean distance between Earth and the Sun, about 93.2 million miles (150 million km). Drawing not to scale.

A small, faint star, Proxima Centauri, is the closest star to our Sun. Half the angle of Proxima's shift is as small as 0.7687 arcseconds or 0.000214 degrees. This "half-angle" is referred to as a star's "parallax". For comparison, Proxima's parallax is equivalent to an object 1 inch in diameter in a distance of 4.23 miles (6.8 kilometers)!

Consequently, the distance to Proxima is the reverse of its parallax: d = 1 / 0.7687 = 1.3 parsecs or 1.3 parsecs * 3.26 = 4.24 light-years.

The unit "parsec" denotes the distance corresponding to a parallax of one arcsecond or 3.26 light-years. Vast distances can be expressed in kiloparsecs, megaparsecs or gigaparsecs.This would look rather clumpsy in light-years.

Greater distances to stars require a wider baseline for obtaining discernible parallaxes. The Earth-Sun baseline is good for parallax angles larger than 0.01 arcseconds. Stellar distances beyond 100 parsecs cannot be measured from Earth with sufficient accuracy.

Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for over 100,000 stars with an astrometric precision of about 0.97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 parsecs away.