Luminous blue variable

Luminous blue variables, often called LBVs, are rare and massive stars.

These stars are highly evolved and show unpredictable changes in their brightness and spectra. They are also known as S Doradus variables. This name comes from S Doradus, a very bright star in the Large Magellanic Cloud. LBVs are among the most luminous objects in the universe. They are unstable supergiants or hypergiants. Their extreme brightness and mass make them critical to understanding how the largest stars live and die.

An LBV undergoes specific physical changes during its life. In a quiescent state, which means a quiet or stable period, these stars are typically B-type stars. During this phase, they can be quite hot. When an LBV enters a normal outburst, its temperature actually decreases to around 8,500 K. This temperature is slightly hotter than yellow hypergiants. Even though the temperature drops, the bolometric luminosity, or total energy output, usually remains constant. However, because the temperature changes, the visual brightness can increase by one or two magnitudes.

Some 3-D simulations suggest these outbursts might be caused by variations in helium opacity. Opacity refers to how much a material blocks light.

There are different types of cycles that these stars exhibit. Some LBVs show strong-active cycles. These are periodic outbursts that can last longer than 20 years or less than 10 years. Other stars show weak-active cycles. These variations are much smaller, often less than half a magnitude. Weak-active cycles always occur on shorter timescales of less than 10 years. Additionally, some LBVs undergo giant eruptions. These eruptions involve much larger increases in luminosity and mass loss. These events can be so violent that they were once mistaken for supernovae.

Astronomers have been studying these unusual stars for centuries. People noticed stars like P Cygni and Eta Carinae in the 17th century. In 1922, John Charles Duncan published the first detection of variable stars in an external galaxy. Edwin Hubble found more in the Triangulum Galaxy in 1926 and 1929. In 1953, Hubble and Allan Sandage conducted a detailed study of several of these stars. They were originally called Hubble–Sandage variables. In 1978, Roberta M. Humphreys linked these stars to the S Doradus class. Finally, in 1984, Peter Conti formally grouped these stars under the term "luminous blue variable."

The scale of an LBV is truly massive. The most luminous LBVs can be over a million times brighter than the Sun. For instance, Eta Carinae reaches 4.6 million times the Sun's luminosity. Their masses are also extreme. Some may approach or even exceed 100 solar masses. The least luminous LBVs have luminosities around 250,000 times that of the Sun. These smaller LBVs have masses as low as 10 solar masses. However, they were likely much more massive when they were on the main sequence. This is because they lose mass very quickly through stellar winds and outbursts.

LBVs are in a very rapid stage of stellar evolution. Their total lifetime is only a few million years. The LBV phase itself lasts much less than a million years.

Models suggest the LBV stage is a way for massive stars to shed excess mass. This might happen during core hydrogen burning or hydrogen shell burning. Some models suggest LBVs are the final stage before a star becomes a Wolf–Rayet star. This is a stage where the star has lost much of its hydrogen. The LBV phase is analogous to the red giant phase seen in smaller stars.

Most scientists believe that nearly all LBVs will eventually explode as supernovae. Some LBVs undergo giant eruptions that create nebulae, which are large clouds of gas. Eta Carinae is a famous example that produced the Homunculus Nebula.

Some of these eruptions are called supernova impostors. This is because they look like a star exploding, but the star actually survives. However, some stars may explode directly as a supernova while in the LBV stage. For example, the progenitor of SN 2005gl was an LBV in outburst just years before it exploded. These stars are a vital link in the life cycle of the universe's most massive objects.