Tag Archives: massive stars

The Betelgeuse hype

Although we are “enjoying” the Christmas break, today I’ve been contacted (thanks to Rami Mandow @CosmicRami) by Kelsie Iorio, an ABC News Digital journalist who was preparing an article about the situation of the red supergiant Betelgeuse.

The article published in ABC News, entitled “Is Betelgeuse, the red giant star in the constellation Orion, going to explode?“, can be found in this link. It includes comments from Rami Mandow, Associate Professor Michael Brown from Monash University’s School of Physics and Astronomy and myself, with some other tweets from several astrophysicists who are talking about the “mysterious dimming of Betelgeuse“.

The red supergiant star Betelgeuse in Orion was the very first star after the Sun we got a direct image of its surface. That is because it is a huge star: if it were where the Sun is its outer layers will reach the orbit of Jupiter! This image was taken with the Hubble Space Telescope in 1995. More info in the APOD (Astronomy Picture of the Day) on April 19th, 1998. Credit: A. Dupree (CfA), R. Gilliland (STScI), FOC, HST, NASA.

But I want to share with you my full interview here, and keep it for my records, so here it goes.

The Betelgeuse hype

Interviewer: Kelsie Iorio  (ABC News Digital )

Interviewee: Ángel R. López-Sánchez (AAO-MQ)

Q: For those who don’t know, what is Betelgeuse and what is it doing at the moment that’s out of the ordinary?

Betelgeuse is a bright star in the famous constellation of Orion the Hunter. It is a red supergiant star, meaning that it is star that is much more massive than our Sun (10 – 15 times) and that is already in the latest stages of its live. The star will eventually explode as a (type II) supernova. We astrophysicists know this will happen “soon”, but “soon” in Astronomy means 100 thousands years or perhaps even more.

During the last month observers worldwide have measured a dimming in the brightness of Betelgeuse.

This is actually completely ordinary, as it is well known and documented that Betelgeuse is a variable star (that means that it’s periodically changing its brightness).

Social media has again played a role here with the “hype” of the brightness of Betelgeuse: it is a normal situation that is happening at the moment and we can explain the dimming of Betelgeuse in many ways (a small shrinking of its huge size, solar spots, magnetic activity, a combination of factors) without the need of thinking that is going to explode now.

But people have been talking about that and many of them would love to see Betelgeuse explode! (I do not).

 

Q: Why is the astronomy world so excited about what’s happening?

If Betelgeuse really explodes as a supernova this would be a great opportunity for use to study how massive stars explode and get a better understanding of stellar evolution and stellar interiors. It will create a point-like object as bright as the Moon that would be visible even during the day, that will be fading during months till disappear.

However I must insist: the dimming of the brightness is the typical behaviour of the star. It is periodically changing its brightness and it has had this “low” brightness in the past. Even Aboriginal Australian knew this star changed brightness!

 

Q: Can we see Betelgeuse from Australia? For people who haven’t seen it or don’t know how to look for it, what does it look like from earth?

Of course! This is a star located very close to the celestial equator, meaning it can be seen essentially from everywhere (just not from the very same South Pole and around). Right now it is clearly visible to the North-East at the beginning of the night. The constellation of Orion is one of the most famous constellations of the sky and can be very easily recognised even by non-experts. Just use a stellar map (there are plenty free in internet, I recommend http://skymaps.com, and also plenty of apps) and you’ll see it. Despite it has noticeable dimmed in the last month, Betelgeuse still is one of the brightests stars in the night sky.

As any other star Betelgeuse is just a point of light, even when using a powerful telescope: stars are very, very, very far away from us to see them like a little disk. Betelgeuse is at a distance of around 700 light years.

 

Q: What do you think will happen to Betelgeuse next? Is it likely to explode?

No, it is very unlikely we see it exploding. The latest astrophysical research conducted about Betelgeuse clearly shows that it should still have a life of around 100 thousand years. Again, that is almost NOTHING in the cosmic scale, but a lot for us.

As it has done plenty of times in the past, Betelgeuse will eventually gain brightness again and all will be back to usual, continuing being sometimes a bit brighter sometimes a bit dimmer during the rest of all our own lifetimes.

 

Q: Is what’s happening with Betelgeuse a rare event?

No, it is not a rare event.

There are PLENTY of variable stars in the sky. A nice example is the star Mira in the constellation of Cetus (the Whale). This star sometimes can be easily seen with the naked eye and sometimes it is imposible to see, needing binoculars or telescopes to detect it. And it is not going to explode as supernova!

 

Q: Do you believe it’s important for the wider public to have a basic knowledge of astronomy and understand what’s happening in situations like this? Why?

I think it is important because everyone loves Astronomy but nowadays it is very easy to be confused because of the mixed bag of content found in social media and the internet.

I could tell the story about why supermoon are NOT a thing, but that is for another time (or read it here: https://angelrls.wordpress.com/2016/11/11/supermoons/)

 

Article in ABC News: “Is Betelgeuse, the red giant star in the constellation Orion, going to explode?“, Kelsie Iorio, 28th December 2020.

The importance of massive stars

The mass range of stars drawing their energy supply from nuclear fusion covers about three orders of magnitude. The least massive stars known have masses around 0.1 solar masses (M) and the most massive examples are around 100 M, although stars with masses of ~150M may also exist.

Massive stars are defined as those stars with masses higher than around 8 M. However, this lower limit is not completely fixed, as the definition of massive star actually comes from those stars that ignite helium and afterwards carbon in non-degenerated stellar cores (i.e., the hydrostatic equilibrium is reached because the inward gravitational force is balanced with the outward force due to the pressure gradient of the gas). Depending on the evolutionary scenario, this happens between 7 and 9 M.

Massive stars consume their fuel faster than low and intermediate mass stars: a solar-mass star has a life ~125 times longer than a 10 M star. Massive stars also are very luminous: a 100M star shines with a luminosity similar to ~1600 Suns. Hence, except for stars of transient brightness, like novae and supernovae, hot, massive stars are the most luminous stellar objects in the Universe.

Young, massive star clusters near the center of the Milky Way, at ~25,000 light-years from Earth: the Arches cluster (left) and the Quintuplet Cluster (right). Both pictures were taken using infrared filters by the NICMOS camera of the Hubble Space Telescope in September 1997. The galactic center stars are white, the red stars are enshrouded in dust or behind dust, and the blue stars are foreground stars between us and the Milky Way’s center. The clusters are hidden from direct view behind black dust clouds in the constellation Sagittarius. Credit: Don Figer (STScI) and NASA.

Massive stars are, however, extremely rare. Following the very famous results obtained by the Austrian-Australian-American astrophysicist Edwin Ernest Salpeter in 1955, the number of stars formed per unit mass interval is roughly proportional to M -2.35. Therefore we expect to find only very few massive stars in comparison with solar-type stars: for each 20M star in the Milky Way there are roughly a hundred thousand solar-type stars; for each 100M star there should be over a million solar-type stars.

However, despite their relative low number, massive stars have a fundamental influence over the interstellar medium and galactic evolution because they are the responsible of the ionization of the surrounding gas and they deposit mechanical energy first via strong stellar winds and later as supernovae, enriching the interstellar medium by returning unprocessed and nuclear processed material during their whole life. Massive stars therefore condition their environment and supply it with new material available for the birth of new generations of stars, being even the triggering mechanism of star formation. They also generate most of the ultraviolet ionization radiation in galaxies, and power the far-infrared luminosities through the heating of dust. The combined action of stellar winds and supernovae explosions in massive young stellar clusters leads to the formation of super-bubbles that may derive in galactic super-winds. Furthermore, massive stars are the progenitors of the most energetic phenomenon nowadays found, the gamma-ray bursts (GRBs), as they collapse as supernova explotions into black holes. Particularly, the interest in hot luminous stars has increased in the last decade because of the massive star formation at high redshift and the results of numerical simulations regarding the formation of the fi rsts stars at zero metallicity (Population III stars), that are thought to be very massive stars with masses around 100 100M.

The descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (105  to 106 solar luminosities, L) O stars are Wolf-Rayet stars, which have typical masses of 25 – 30 M for solar metallicity.

What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (105  to 106 solar luminosities, L) O stars, with masses 25 – 30 solar masses (M) for solar metallicity. WR stars possess very strong stellar winds, which reach velocities up to 3,000 km/s. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV ranges. These strong winds are also attributed to atmospheres in expansion. Actually, these winds are so strong that they are peeling the star and converting it in a nude nucleus without envelope. Indeed, WR stars have ejected their unprocessed outer Hydrogen-rich layers. WR stars typically lose 10−5 M a year; in comparison the Sun only loses  10−14  M⊙  per year.

Hα image of the Population I Wolf-Rayet star WR 124 (WN8) showing a young circunstelar envelope that is ejected at velocities highest than 300 km/s. The chaotic and filamentary structure created forms the M 1-67 nebula. The star is located at about 4.6 kpc from the Sun. At the left, image obtained by the author using the IAC-80 telescope, combining filters Hα (red) Hα continuum (green) and [O III] (blue). The right Hα image was obtained by the Hubble Space Telescope WFPC2 (Grosdidier et al. 1998). Note that the large arcs of nebulosity extend around the central star yet with not overall global shell structure. Furthermore, numerous bright knots of emission occur in the inner part of the nebula, often surrounded by what appear to be their own local wind diffuse bubbles. The dashed square in the IAC-80 image indicates the size of the HST image.

This is Figure 2.1 in my PhD Thesis.

WR stars were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. They found that three bright galactic stars located at Cygnus region have, rather than absorptions lines, broad strong emission bands superposed to the typical continuum of hot stars. In 1930 C.S Beals correctly identified these features as emission lines produced by high ionized elements as helium, carbon, nitrogen and oxygen.  The intriguing spectral appearance of WR stars is due both their strong stellar winds and highly evolved surface chemical abundance. In 1938, WR stars were subdivided into WN (nitrogen-rich) and WC (carbon-rich) depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen , respectively. Not until the 1980s did it became clear that WR stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. 

The mass-loss occurs via a continuous stellar wind which accelerated from low velocities near the surface of the star to velocities that exceed the surface escape speed. Their spectra, originated over a range of radii with the optical continuum forming close the stellar core and the emission lines in the more external areas (even beyond 10 stellar radii), indicate that the WR stars are embedded in luminous and turbulent shells of ejecta owing outwards at speeds comparable to the expansion velocities of novae although, in the case of WR stars, the expanding shell is being constantly fed with material from the main body of the star.

WR stars are extremely rare, reflecting their short lifespan. Indeed, they live for only some few hundred of thousands years, and hence only few WR stars are known: about 500 in our Milky Way and 100 in the Large Magellanic Cloud. Indeed, because of their peculiarities (brightness and broad emission lines), WR stars can be detected in distant galaxies. A galaxy showing features of WR stars in its spectrum is known as a Wolf-Rayet galaxy.

I compiled the main characteristics of WR stars in Chapter 2 of my PhD Thesis. A recent review about the properties of WR stars was presented by Crowther (2007).