Stars

The Brightest Stars


Number
Common
Name
Constellation
Apparent
Magnitude
Spectral
Type
Luminosity
(Sun = 1)
Distance
(Light Years)
Radial
Velocity
(km / sec)
1 Sirius Canis Major -1.46 A1 26 8.7 -8
2 Canopus Carina -0.72 F0 15,000 310 +21
3 Alpha
Centauri
Centaurus -0.04 G2 1.7 4.3 -22
4 Arcturus Boötis 0.00 K2 115 36 -5
5 Vega Lyra 0.03 A0 52 25 -14
6 Capella Auriga 0.08 G8 F0 90 70 43 +30
7 Rigel Orion 0.12 B8 60,000 910 +21
8 Procyon Canis Minor 0.38 F5 7 11.4 -3
9 Achernar Eridanus 0.46 B5 400 85 +19
10 Betelgeux Orion 0.0 - 0.9 M2 105,000 v 640 +21
11 Agena Centaurus 0.61 B1 10,000 460 -11
12 Altair Aquila 0.77 A7 10 16.6 -26
13 Acrux Crux Australis 0.83 B1 3,200 360 -11
14 Aldebaran Taurus 0.85 K5 120 68 +54
15 Antares Scorpius 0.96 M1 7,500 330 -3
16 Spica Virgo 0.98 B1 2,100 260 +1
17 Pollux Gemini 1.14 K0 60 36 +3
18 Fomalhaut Piscis Australis 1.16 A3 13 22 +7
19 Deneb Cygnus 1.25 A2 70,000 1,800 -5
20 Becrux Crux Australis 1.25 B0 8,200 425 +20


Explanation

Number

This is a list of the 20 brightest stars as seen from the Earth (not including the Sun). The stars are numbered from 1 to 20 in sequence.

Common Name

This is the name by which the star is commonly known. The names are Greek, Latin or Arabic. This web site is based in London: stars not visible from London are in red.

Some examples of the names: Deneb is Latin for tail (because it marks the tail of The Swan - Cygnus); Antares is Greek for rival of Mars (because of its red colour); Aldebaran is Arabic for eye of the bull (because it marks the eye of The Bull - Taurus).

Constellation

A constellation is a star group (as seen from Earth) that the star is a part of. Constellations are human inventions. The stars in them appear in the same part of the sky but are, in fact, at different distances from us and not related to each other. Different cultures use different constellations. For more, read Astronomy and Astrology.

In the West, there are 88 recognised constellations; 48 of these date from Roman times and are known as the Classical Constellations. These include the 12 Zodiac constellations through which the Sun, Moon and planets always pass through. Constellations are always known by their Latin names.

Some examples: Canis Major means The Great Dog; Orion is The Hunter; Crux Australis means The Southern Cross.

Constellations are used by astronomers for convenience. We say that Sirius is in Canis Major rather than give its celestial coordinates.

Apparent Magnitude

Apparent Magnitude tells how bright the star is as seen from the Earth. The magnitude scale was devised by the Ancient Greeks. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.

In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.5 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. On this scale, Jupiter has a magnitude (at its brightest) of -2.6, Venus is at -4.4 and the Sun -27. The faintest stars visible to the naked eye are sixth magnitude. Pluto has a magnitude of +14, far too faint to be visible without a powerful telescope.

In the table it can be seen that Betelgeux varies its magnitude - some stars are variable in brightness.

The brightness of a star as seen from Earth depends on its intrinsic luminosity and its distance from Earth. A dim star may appear bright because it is close while a luminous star may appear faint because it is far away. This is why we say Apparent Magnitude.

Spectral Type

When starlight is passed through a prism, it splits into its constituent colours, like a rainbow. This is called the star's Spectrum. Stellar spectra are crossed by dark lines. These lines give astronomers a lot of information about the star: temperature, luminosity, radius, magnetic properties, movement. Read The Electromagnetic Spectrum for more on spectra.

Stellar spectra are classified into types. These types are given letters. The spectral type series is a temperature series. Moving from the hottest stars to the coolest, the series of letters runs O, B, A, F, G, K, M.

Each spectral type is subdivided into ten numbers. For example, A0, A1, A2, up to A9. A0 is hotter than A1. The table below gives more information.

Spectral
Type
Colour
Surface
Temperature
(°C)
O
Blue >30,000
B
Blue-White 20,000
A
White 10,000
F
Yellow-White 7,000
G
Yellow 6,000
K
Orange 4,500
M
Red 3,000

Our Sun is a star of Spectral Type G2 with a surface temperature of around 6,000°C.

Luminosity

This tells us how much more energy and light the star gives off compared with the Sun. This is how bright the star really is once distance has been taken into account. There is a huge variety in the luminosity of the stars. At one extreme, the star Alpha Centauri is 1.7 times more luminous than the Sun. At the other extreme, Canopus is 15,000 times more luminous than our Sun.

Luminosity can be measured indirectly by combining the apparent brightness of a star with its distance. It can also sometimes be measured directly from the spectrum.

Distance

The distance of a star is given in Light Years. This is the distance covered by a light beam in one year. Light travels at 300,000 km per second (186,000 miles per second). In one year a beam of light will travel 9.4 million million km (5.9 million million miles). This enormous distance is a Light Year.

Many stellar distances can be measured directly by trigonometry. As the Earth moves around the Sun, the star appears to shift its position against more distant stars. This effect is called parallax. It is a tiny effect but can be measured. The amount of the parallax depends on the diameter of the Earth's orbit around the Sun (just under 300 million km or 186 million miles) and the distance to the star. A star with a paralax of 1 second of arc (written 1") is said to be at a distace of 1 Parsec. 1 Parsec is equal to 3.26 Light Years.

Other stars can have their luminosity measured by their spectra or by other properties. When this is compared to their apparent brightness, a distance can be calculated.

For more on astronomical distances look at The Scale Of The Universe.

Radial Velocity

This the velocity of the star relative to the Sun. Negative velocities denote a star moving towards the Solar System. Positive velocities are for stars moving away from us.

Radial velocity is easily measured by looking at the star's spectrum. The lines on the spectrum are shifted to the blue end if the star is moving towards us (the so-called blue shift) and to the red end if the star is moving away from us (red shift). The amount of this shift depends on the relative velocity between us and the star.


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KryssTal Related Pages

Tables with explanations about the properties of Solar System bodies like the Sun, Earth, Moon, planets, asteroids and comets.

An account of how various properties of stars can be measured by studying starlight. Includes brightness, distance, luminosity, temperature, mass, radius, density and an introduction to the H-R Diagram.

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