Is Betelgeuse about to become a supernova?
Figure 1: Betelgeuse’s dust plumes seen by VISIR image
Betelgeuse (α Orionis; HD 39801) is a nearby
luminous red supergiant (M1-2 Iab-a) located approximately 200 parsec from the Sun, is most likely destined
to explode as a classic Type II-P supernova (SN II-P) and leave behind a neutron star. Study of Betelgeuse thus promises insight into a broad range of issues of the structure, evolution,
rotation, magnetic fields, mass loss, stellar winds, circumstellar medium, dust formation, atmospheres,
chromospheres, radiative transfer, nucleosynthesis, and, eventually, the explosion of massive stars. Betelgeuse is easy to identify in the constellation Orion. Since October 2019, it began to dim
noticeably, and by January 2020 its brightness was reduced 2.5 times, from magnitude 0.5 to
1.5. Stimulated by mysterious dimming, many intensive observations in various
wavelengths, and detailed analysis
has been performed. These
investigation suggests that effective temperature decreased by ∼ 100 K during the
Great Dimming, and a substantial mass ejection possibly
occurred.
1.1
Facts about Betelgeuse
This red super giant, at a distance of 222 ± 40 pc, has an age of 8-8.5 Myr and it has expanded to a size of 887 ± 203 R⊙. However it has cooled to a temperature
≈ 3600 K.
Figure 2: This is the first direct image of a star other
than the Sun, made with the Hubble Space Telescope called Betelgeuse
Due to expansion and hence conservation of angular momentum, giants tend to rotate slowly. Betelgeuse spins once every 8.4 years, whereas the
Sun spins once a month. Luminosity is directly proportional to the radius R and temperature T of a star and is given by:
L = σ4πR2T 4 (1)
Though it’s temperature is low, due to the large size, the luminosity is 125,000
×L⊙. Betelgeuse has used up the hydrogen fuel in it’s core and is presently fusing helium into carbon,
hence generates lots of heat.
2
Evolution of Betelgeuse
Normally stars fuse hydrogen to helium most of their lifetimes is known as main sequence. There are some types of common nuclear
reaction: [4]
4H1
−→ 4He + 2e + 2γ + 2νe (2)
CNO Cycle(Carbon-Nitrogen-Oxygen):
4H1 + 2e + (CNO) −→ 4He + 2e +
2γ2νe + (CNO) (3)
Triple-α:
3He4 ←− 12C + γ (4)
Figure 3: Variability of brightness with time
The
kind of nuclear
fusion reactions take place completely depends on it’s core
temperature. For a burning star nuclear reaction is mostly dominated by Proton-
Proton(PP) chain reaction, which is less efficient and denotes longer lifetime for low
mass stars.
2.1
Pulsation Periods [1]
During the main sequence phase, any star balances the
pressure generated by energy release and the weight of the star. After completion of the core fuel outward
radiation pressure decreases. As a
result gravitational collapse actually increases temperature and forces
the heavier
elements to fuse. The helium core grows on the red super giant and the radiative envelope expands. The stars which
are no longer in a hydrostatic equilibrium, often shows variability in
pulsation period. So Betelgeuse is
classified as the pulsating red super giant. Studies of Betelgeuse have long revealed a variety of
pulsation modes. It has been estimated
that ∼ 105
years later Betelgeuse will
explode as Type II supernova, releasing 2 × 1053 erg neutrinos and leaving behind neutron
star of mass ∼ 1.5 M⊙.
3
The Great Dimming
Betelgeuse
surprised the astronomical community and fascinated people worldwide when it went through a phase of anomalously low optical luminosity beginning in October 2019 and lasted
till March 2020.
Guinan et.al. (2020)
and Levesque & Massey (2020) thus framed the extremes
of the possible explanations of the
Figure 4: Resolved images of Betelgeuse through the Great Dimming
Great Dimming
that continues to be debated. Early discussions about effective temperature during the dimming
assumed to be spherically symmetric. Later
experiments with Spectro polarimetry provided new methods
to explore the geometry of the surface of Betelgeuse.
The Great Dimming was clearly
a complex phenomenon. It seems unlikely
that its alignment with the phase
of the ∼ 400 days pulsation period was a coincidence, but that alone cannot account
for the magnitude
and spatial inhomogeneity of the obscuration. There seems to be solid evidence that dust with an inhomogeneous spatial distribution played a role. A decrease in temperature occurred, perhaps in the form of large, cooler
patches. Such patchy structure in surface temperature calls into question the meaning of a global effective temperature and at least sets upper limits
on the precision with
global effective temperature can be determined.
4
Future [2]
Betelgeuse still presents
a host of outstanding issues.
Its irregular surface; the manner in which it ejects
matter to form a chromosphere, wind, dust, and molecules; how it came to move
through space and spin so rapidly; the nature of its variability and magnetic
fields. About 600 years later,
detection of those neutrinos will give any humans on Earth their first hint of the
events to come, i.e.
∼ 100 trillion neutrinos, vastly less than a lethal dose of radiation. At the Earth,
Betelgeuse will appear as a pinpoint about as bright as a quarter Moon lasting for around three months. The explosion will then fade, but remain visible at the Earth for many years and to scientific instruments for centuries. But till then, the star closest to going supernova is Eta Carinae which is is a two-star system about 10 times further away that’s already showing signs for decades now and it will light up our skies in the daytime.
References :
The evolutionary stage of Betelgeuse inferred from its pulsation periods
Will Betelgeuse explode in ‘tens of years’ ?
A closer look at the pp-chain reaction in the Sun
- Soham (MS22119)
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