Alpha Crucis

by Alice Han

The 'Alpha Crucis' is better known as 'Acrux'.

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What is an 'Alpha Crucis'?

It is 'a' star that is part of the constellation Crux (Southern Cross; see picture above). There are four other stars named Crucis--Beta Crucis, Gamma Crucis, Lambda Crucis, and Mu Crucis. Out of the five, the Alpha Crucis is the brightest. But, scientists discovered that It is the brightest because it is actually a three-star system--three stars make up Acrux. The main star is named Alpha-1, or Alpha A, which is actually two stars (the other one is Alpha 1.2), and the other is Alpha-2. But the main one is Alpha-1.1, which is what we'll be talking about.

The Acrux is the 13th brightest star in the night sky.

Basic Information

  • The distance from Earth to Acrux would be 325.8 light years.
  • 28,000 kelvins is its temperature.
  • Its luminosity is 2 times 10 to the 31st power. (25,000 L☉=25,000 times the Luminosity of the sun)
  • Its mass is 34 times 10 to the 31st power. (14 times the mass of sun)
  • When it dies, it will either be a neutron star or a black hole.
  • The apparent magnitude is positive 0.77
  • The absolute magnitude is negative 4.23
  • Spectral Class is B, almost O. (Blue-white subgiant)
  • It is in the red supergiant stage at this point.

Acrux belongs to the constellation 'Crux', aka Southern Cross, located in the southern hemisphere.

Science Definitions for Dummies

  • Light Year is a unit of astronomical distance equivalent to the distance that light travels in one year.
  • Luminosity is generally understood as a measurement of brightness. In this case, luminosity measures the total amount of energy emitted by a star.
  • Apparent Magnitude is what is visible to the naked eye.
  • Absolute Magnitude is when the stars are at a standard distance, and they calculate the magnitude from there by comparing it with other stars.
  • Spectral Class is determined by the temperature of the star's surface. For more info, scroll down below...


What is a Hertzsprung-Russell Diagram?

A Hertzsprung-Russell Diagram was made to classify stars the EASY way. The diagram below shows you the diagram. As you can see, Spectro Class O is of around 40,000 Kelvin in surface area. Acrux is a Spectro Class B, but almost O. So its surface temperature is in the range of 20,000 and 40,000 kelvin.

Also, what is significant about this graph is that it goes backwards, highest to lowest, left to right. It is set up like this because it is easier to observe the patterns and such. As you can see in the bottom, you can see the line of stars going almost diagonally. Those are in the main sequence, and they are the most stable.

The ones that have low luminosity but high in temperature, they are the White Dwarfs (bottom left of graph)

The ones that are all clustered up in the middle-ish point of the graph are the Red Giants, which are low in temperature but high in luminosity.

The ones that are both hot and cool and high in luminosity, those are called the Super Giants. (The ones that are slightly spread out up at the top)

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This is where Acrux is located in the diagram. (below)
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  • "Acrux (Alpha Crucis)." Acrux (Alpha Crucis). N.p., n.d. Web. 19 Sept. 2014.

  • "Acrux." - Conservapedia. N.p., n.d. Web. 22 Sept. 2014.

  • "The History of the Star: Acrux." Acrux. N.p., n.d. Web. 22 Sept. 2014.

  • "Alpha Crucis (star)." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 22 Sept. 2014.

  • "Hertzsprung-Russell Diagram." H-R Diagram. N.p., n.d. Web. 23 Sept. 2014.

  • "Acrux." Big and Giant Stars:. N.p., n.d. Web. 23 Sept. 2014.

  • "Take Wolfram|Alpha Anywhere..." Acrux. N.p., n.d. Web. 23 Sept. 2014.