10 Interesting Facts about Blue Giant Stars

Alnilam in Orion
Image Credit: Alnilam in Orion by Philip A. Cruden

Since there is no clear definition of blue giant stars, the term is frequently applied to any hot, massive star, albeit erroneously in some cases. For the most part, though, blue giant stars fall into the O and B spectral classes, and are categorized as either luminosity class III giants or class II bright giants.

While blue giant stars have a surface temperature of at least 10,000 Kelvin, compared to say a yellow dwarf star like our Sun at about 6,000K, another type of star called blue supergiants (class I) are even more extreme, with a surface temperature of between 10,000–50,000K and luminosities of 10,000 to a million times brighter than the Sun. One famous example is Rigel in the constellation of Orion, which is a class B supergiant that is 25 times bigger than the Sun, and has a surface temperature of 11,000K. Refer to the image below; here blue giants are represented by the giant stars Bellatrix and Spica, while the blue supergiants Rigel and Deneb appear to the upper right of the main sequence.

Below are 10 more interesting facts about blue giant stars you may not have known.

Blue giant stars are not a stellar class

In astronomy, the term “blue giant star” does not have a clear definition. In practice, a blue giant star can be in any one of a variety of evolutionary star states, with about the only common aspects between them being that they have all evolved off of the main sequence, and that they inhabit a specific area of the H-R diagram, i.e., to the upper right of the main sequence. However, even though the term “blue giant” is not clearly defined. It is often erroneously applied to some hot and massive stars such as Wolf-Rayet stars, simply because these stars are big and hot.

Blue giant stars are relatively small

Despite their giant status, blue giants are only moderately bigger and more luminous than they were when they were on the main sequence. Nonetheless, with minimum temperatures of 10,000K, these stars are hot enough to emit blue light, which places them in the O, B, and sometimes earlier as spectral classes. Typically, a blue giant star would have an absolute magnitude of about 0 and brighter, and be about twice as massive as the Sun, while typically being only about 5 to 10 times bigger.

Heaviest blue supergiant is 315 times more massive than the Sun

While blue giant stars are typically more modestly sized, blue supergiant stars can have more than 25 solar radii and 20 solar masses, making them the most massive stars in the Universe. The blue supergiant star found in the Large Megallanic Cloud designated R136a1, for instance, is so massive that its very existence is posing a serious challenge to all the standard models of star formation.

At 29 times bigger than the Sun, R136a1 is not the largest star yet found, but it is the most luminous, shining at a whopping 8.7 million solar luminosities with its incredible surface temperature of about 53,000K. It also has somewhere between 265 and 315 solar masses, making it the most massive star yet discovered. However, the star is blowing off its own mass at a rate about 20 billion times that which our Sun is shedding its own mass every year, and it is estimated that R136a has lost about 50 solar masses since its birth about 800,000 years ago.

Hertzsprung-Russel diagram
Image Credit: ESO

Blue giant stars can switch colors

While massive stars expand when hydrogen burns in a shell around their mainly helium cores, they do not gain much luminosity as they move horizontally across the H-R diagram. In practice, this means that a massive star can rapidly change from being a blue giant to becoming a bright blue giant, and then a yellow supergiant, before ending up as a red supergiant. The luminosity class of such a rapidly changing star is determined by the changes in its spectrum that are caused by changes in temperature and surface gravity.

Most blue giants occur in OB associations

Most blue giant stars fall into the O spectral class, and most of them occur in OB stellar associations, which are small clumps of hot and massive stars that are thought to have originated at about the same time, and from the same molecular cloud. Once the fully formed stars have blown away the remaining gas and dust, the tightly bound O and B-type stars become unbound and start to drift apart. Note though that a typical OB association will also contain hundreds and sometimes thousands of stars of other types as well.

Blue giant stars are very short-lived

Because of their relatively high masses, blue supergiants of the O spectral class will burn through their hydrogen fuel in only about a million years or so, before expiring as supernovas a few million years later. As a result, the average age of OB associations is only a few million years, and most associations will lose all their O and B-class stars in less than 10 million years.

The fastest known rotator is a blue giant star

Blue giant stars are extreme in many ways, one example of which is the rotational velocity of VFTS 102, a 25-solar mass blue supergiant star in the star-forming region of the Large Magellanic Cloud called the Tarantula Nebula. Studies have shown that the star is rotating at about 600 km/sec (100 times faster than the Sun) at its equator, which is so fast that material is being flung off the star to form a disc of stellar material around it. While the exact reasons for this high rotation rate is not certain, it is thought that the star is being “spun up” by the accretion of material from a close companion star.

Supernova SN 1987A was the death of a blue giant star

Designated SN 1987A because it was the first supernova observed in the year 1987, this supernova had the blue supergiant star Sanduleak -69° 202 as its progenitor, which was rather surprising since at the time, it was thought by most investigators that blue giants of any type cannot produce supernova events. Nonetheless, the supernova’s blue-supergiant origin was confirmed a few months after the event when photographs of the area showed that the star Sanduleak -69° 202 had disappeared.

Blue giants are the likely progenitors of most black holes

Unlike red giant stars that are big because they are swollen, blue giants are big because they contain a lot of material. Thus, when blue stars die, their cores are so big that they are thought to be unable to support themselves against gravity through the repulsive forces of neutrons, which means that the core will keep on collapsing until it forms a black hole. However, not all black holes are formed by blue giant stars, but the most massive blue supergiants will almost certainly form black holes when they die.

Blue giant stars are rare

Although blue giant stars are among the rarest of stars, they are among the most luminous in the sky, meaning that many of the brightest stars in the sky are blue giants, despite their rarity. A famous example of a bright blue giant star is Spica, a binary system whose primary component is a blue giant, which together with its companion, make up the 4 brightest stars in the constellation Virgo. Moreover, Spica was the star whose movement across the sky had led the ancient astronomer Hipparchus to discover the precession of the equinoxes.

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