The reason why it's said that it is "usually" the eighth brightest star is its apparent magnitude varies between 0.2 and 1.2. At its brightest it is the seventh brightest star, being brighter than Procyon and when at its dimmest it is about the 19th brightest.
Betelgeuse is a type M red supergiant and is a very young star, being less than 10 million years old. In comparison, the Sun is an estimated 4.6 billion years old. It is also one of the most luminous and largest stars known. If Betelgeuse were situated where the Sun is, its photosphere would reach out to at least the asteroid belt and quite possibly beyond Jupiter.
Betelgeuse's mass is why it is a red supergiant yet still an extremely young star; this also makes it a prime supernova candidate. The mass is not known with anything approaching certainty, as a companion is required to theoretically measure it, but estimates range between 5 and 30 solar masses (a solar mass is the mass of our own Sun).
Betelgeuse is travelling through the galaxy at the high speed for a star of 30km/second away from its origin in the Orion OB1 stellar association. Stellar associations are groups of stars that share a common origin and can be considered a very loose star cluster. The members of an association are not gravitationally bound like a normal star cluster, but are instead usually drifting - or speeding - away from each other.
The Orion OB1 association includes the three stars of Orion's Belt, the stars that make up the Sword and thousands of others, a small number of which are also young, hot giant stars like Betelgeuse used to be (it is now comparatively cool) and those of the Belt and Sword. The OB in the name means that the group will contain 10-100 stars of the spectral types O and B. Type B stars are blue in colour, type Os are bluish but the majority of their output is in the ultraviolet range.
There are various shells of ejected matter surrounding the star itself, comprised of different types of matter ejected from the star which also both absorb and emit light, making it hard to see the photosphere. These may also contribute to the variability in the apparent magnitude and the shifting shape of Betelgeuse. There may even be stellar companions inside the surrounding nebula.
A variable star is a star whose apparent magnitude, which is its brightness as seen from Earth, varies. Most stars - including the sun - vary in magnitude to at least some degree, although with the Sun it usually only varies by about 0.1% over an eleven year period. The change in apparent magnitude can be because something gets between the star and the Earth, or because their actual luminosity changes.
Betelgeuse is a star whose luminosity changes and is classed as a semi regular star of subgroup SRc. Semi regular variables are usually red supergiants or giants. Although they may show regularity in how their luminosity changes, they also go through periods of irregularity.
The current accepted theory is that is that the outer layers of the supergiant expand and contract, changing the apparent magnitude and the size of the star. Red supergiants are also believed to have several, giant convection cells as opposed to the many, smaller ones that a star like the Sun would have.
It is not absolutely certain that the expansion and contraction, giant convection cells and the surrounding shells of matter are what causes the variability and size change of Betelgeuse, although it seems the most likely. Alternatively, it could be that the star is just extremely unstable and on the verge of becoming a supernova.
What is a Supernova?
The term "supernova" was first coined by Walter Baade and Fritz Zwicky by adding the word "super" to the front of "nova" (Latin for "new"). The term nova was used to describe what appeared to be a new star and is used to refer to a smaller magnitude explosion in a white dwarf star. The "super" is added to the front to distinguish the two apart.
Supernova can easily be detected as they can outshine their entire home galaxy and, if they happen close enough, can be easily be seen with the naked eye. The supernova SN 1604 could be seen with the naked eye and was 20,000 light years away, so close enough is a pretty relative term.
Supernovae are caused either by a degenerate star (such as a white dwarf) suddenly reigniting in a thermal runaway supernova. This can happen if a white dwarf gains enough material from a companion star to reignite, or when two white dwarf stars merge. A white dwarf is an extremely dense stellar remnant, thought to be formed after the death of a star which was not massive enough to produce a neutron star. The likelihood of the merger of two white dwarves is actually quite high; binary star systems are very common, so given that the white dwarf is the most likely outcome for a star at the end of its life, it is entirely possible to find systems with two white dwarf stars in them.
The other way is if a suitably massive star at the end of its life, such as Betelgeuse, experiences gravitational collapse, triggering a core collapse supernova.
Supernovae are actually classified by the light emitted by them, rather than by their cause, so two supernovae with the same cause could in theory actually be of two different types.
There are two main types of supernovae; Type I supernovae which have no hydrogen in their absorption lines and Type II which do. Each of these types is further refined into various sub-types depending on other absorption lines present and the shape of the supernova's light curve. Absorption lines in light show what elements are present; the light curve is a graph of the supernova's apparent magnitude.
A supernova event may result in a number of different effects other than the explosion itself. Degenerate remnants such as neutron stars, pulsars and black holes would not be expected to be produced by a Type 1a supernova because these are caused by white dwarf stars.
Gamma Ray Burst
A gamma ray burst (GRB) is the brightest event of electromagnetic radiation known. They start with a burst of gamma rays which can last from milliseconds to minutes, and then fade away to longer wavelengths of the electromagnetic spectrum such as X-rays, then ultraviolet light and continuing to proceed through the spectrum. Gamma rays are a high frequency and energy type of electromagnetic radiation and are extremely dangerous and can cause radiation sickness, DNA damage, cell death and burns, and thus death of the effected being.
Supernovae are one of the theorised causes of gamma ray bursts which occur when a high mass star at the end of its life collapses to form a neutron star or a black hole. The energy would be released as a GRB along the axis of rotation of the star in the supernova. The GRB would then travel in a straight line from that.
Should a GRB from a source relatively close to use - such as in our own galaxy - hit the Earth, the effects on the biosphere could be catastrophic. Some of the mass extinctions in the past have been theorised as having been caused by GRBs.
Neutron stars are a possible stellar remnant resulting from a supernova. They are called neutron stars because they consist almost entirely of neutrons, an uncharged subatomic particle slightly more massive than a proton (which has a positive charge).
Neutron stars are extremely hot, small - in the region of 12km in diameter - and massive, having between 1.4 and 3.2 solar masses, thus weighing more than the Sun does yet be smaller in diameter than even many asteroids.
The beam is emitted from the magnetic poles of the pulsar. Magnetic poles rarely match up with the rotational poles, so the beam sweeps across the sky as the pulsar rotates. The intervals between the pulses emitted range from milliseconds to seconds and can happen with a precision only rivalled by atomic clocks.
At the event horizon of a black hole, matter and light can only pass inwards towards its mass, which means that the only way of detecting a black hole is by its interactions with its environment outside the event horizon.
Naked Eye Supernovae
Although many supernovae are detected, only a few have been visible to the naked eye. The majority of supernovae happen in other galaxies and usually require equipment to detect. Those that are visible with the eye alone are usually comparatively close.
Supernovae are named by the abbreviation SN, followed by the year in which they happened which in this case is 1006 AD. As observational instruments improved multiple supernovae could be detected in one year, so more recent supernovae have letters appended to the name, a process which started in 1885 and following which all supernova have a letter appended, even if only one is detected during the year. Initially, uppercase letters are added for the first 26, then pairs of lower case letters from then on.
SN 1006 is the brightest apparent magnitude event in recorded history. One of the best descriptions of the event was by astronomer Ali ibn Ridwan who wrote "...spectacle was a large circular body, 2½ to 3 times as large as Venus. The sky was shining because of its light. The intensity of its light was a little more than a quarter that of Moon light."
There were multiple other sources referencing the supernova, including Chinese and European; the latter including a report from Switzerland where the supernova would have barely risen much over the horizon. The light from the supernova at its brightest would probably have been enough to read by.
SN 1006 happened at a distance of 7,200 light-years from the Earth and its' peak magnitude was -7.5. The supernova remnant of SN 1006 was discovered in 1965.
SN 1006 is not the only supernova in recorded history that was seen by the naked eye (there are about eight total), or even the closest, but it was the brightest. Some of the others were SN 1054, SN 1572 and SN 1604, all of which were also bright.
SN 1572, also known as Tycho's Supernova after the Danish astronomer Tycho Brahe, even though he wasn’t the first to discover it, because he published an extensive work about the supernova in 1573. The supernova happened of a distance of 8,000 to 9,800 light-years and reached a peak magnitude of -4. Many accounts and observations of the supernova exist.
SN 1604 is also known as Kepler's Supernova after German astronomer Johannes Kepler. Again, as with Tycho's Supernova, Kepler was not the first to observe it but again he published a comprehensive work on the supernova which he observed for an entire year. SN 1604 happened at a distance of 20,000 light years and reached a peak magnitude of between -2.25 to -2.5. This was also the last supernova that was definitely observed with the naked eye with an origin within our Milky Way galaxy.
Near-Earth supernovae are those that happen close enough to the Earth to affect our biosphere in ways that are potentially catastrophic. Supernova that occur up to an estimated up to 100 light-years away could damage the Earth, although this could extend up to 3,000 light-years or so. Type 1a supernovae are expected to be the most likely to cause damage to Earth, but none of this is known for certain one way or the other.
Why is Betelgeuse Dangerously Close?
SN 1006 was more than ten times further away than Betelgeuse currently is. Light, and other forms of electromagnetic radiation including the lethal gamma ray, obey the inverse-square law so being ten times closer does not mean a mere tenfold increase in intensity, but much, much more.
When Will Betelgeuse Blow?
This is not currently known. The mass of a star is an important factor in when a star goes supernova, and the mass of Betelgeuse is not accurately known. Despite Betelgeuse being a young star, it is actually quite old for its size, and a supernova is expected comparatively soon. Comparatively soon means that it is expected to happen within the next million years. Note that this does not mean it will happen in a million years, it could happen in two million. Or 500,000. Or it could have already gone supernova and the blast front is expanding towards us.
Research is being done into determining the mass of Betelgeuse in order to more accurately predict the event.
What Adverse Effects Will it Have?
There is also always a possibility that by the time Betelgeuse explodes humanity has reached and spread out amongst the stars, including locations at more risk of being vulnerable to the effects of Betelgeuse going supernova.
The probable, and preferable, outcome is that the supernova will just be an incredibly bright star with an estimated apparent magnitude of -12 that is visible during the day and outshines the full Moon at night.
IK Pegasi is not expected to go supernova for several million years, at which point the system will have travelled further away from the Solar System and will therefore be less of a threat.
The binary system Sirius is even closer at a mere 8.6 light years and is also a binary system comprised of an A class star and a white dwarf companion but it is not, fortunately, considered a supernova candidate.
Until technology improves allowing better observation of stars and distant supernovae, we cannot tell what is actually happening when a star explodes. Much of what is believed would happen if a supernova happened relatively close to the Earth is theoretical, as no naked eye supernova- which are the closest - have been observed in our own galaxy with anything close to modern instruments. All the ones seen have been a very long way off.
All in all, a supernova is the sort of event that is best viewed from a safe distance. Such as in another galaxy.
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