Is Betelgeuse about to become a supernova?

Figure 1: Betelgeuse’s dust plumes seen by VISIR image

1 Introduction

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.