- 3685 - BETELGEUSE - a star ready to explode? By plotting changes in the brightness of the red supergiant star Betelgeuse we witness the titanic mass ejection of a large piece of its visible surface. The escaping material cooled to form a cloud of dust that temporarily made the star look dimmer, as seen from Earth.
--------------------- 3685 - BETELGEUSE - a star ready to explode?
- Betelgeuse is a star that is now beginning to slowly brighten. Betelgeuse is a very different star from our Sun. While our Sun is a main-sequence star in its prime of life, Betelgeuse is a ‘red giant star’s on the verge of death. But the death of a star is not a simple process.
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- Stars shine so brightly and for so long because of a delicate balance of gravity and nuclear fusion. Gravity would like to collapse a star under its weight. Without nuclear fusion, gravity would crush a star into a white dwarf, neutron star, or black hole.
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- But the crushing pressure gravity creates allows hydrogen in the star’s core to fuse into helium. The process is known as the proton-proton chain and combines four hydrogen nuclei into one helium nucleus. About 3% of the original mass is converted to energy in the form of gamma rays. This energy heats the core even further, letting it push back against gravity.
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- For stars larger than the Sun, another fusion process known as the “CNO cycle” kicks in. CNO stands for Carbon-Nitrogen-Oxygen because the process fuses helium into those three elements. This process is why those three elements are the most abundant in the universe except for hydrogen and helium.
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- While both the “pp-chain” and “CNO cycle” can occur at the same time within a star, over time the CNO cycle increases as hydrogen become more scarce and helium more abundant. Since the CNO cycle releases more energy at a faster rate than the pp-chain, this means a star’s temperature increases over time. We see this gradual heating in our own Sun. By the time the CNO cycle dominates in a star, it’s core is so hot that the outer layers of a star swell and expand.
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- This is the stage Betelgeuse is in now. For millions of years, it was a main-sequence star of about 20 solar masses. But it is now fusing helium so furiously that it has bloomed into a “red super giant“. Betelgeuse is running out of fuel, and in the end, gravity will win. It’s only a matter of time.
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- Betelgeuse before and after its unprecedented dimming taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed.
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- Betelgeuse has enough helium to stay in the red supergiant stage for about 100,000 years. Even after it runs out of helium, it will be able to fuse carbon into heavier elements for about a millennium. After that things will change fairly quickly. When it runs out of carbon it will try fusing heavier and heavier elements for about a year. Then its core will collapse, Betelgeuse will become a supernova.
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- Betelgeuse is still deep in the red supergiant phase of its life. Even though it has dimmed significantly of recent, it isn’t on the verge of exploding. The gradual dimming and brightening we see suggest that it won’t be exploding in our lifetimes. It suggests that the core of Betelgeuse is still chugging away at a steady pace.
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- The changing brightness of Betelgeuse is due to a process known as convection. The upper layers of the star are heated by the core, and this generates a flow of hotter and cooler regions. Material in the interior is heated and rises to the surface. It then cools and sinks into the star, and the cycle continues.
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- Convection happens in the outer regions of most stars, including our Sun. On the surface of the Sun, these convection regions are known as granules, and they are typically the size of Texas. That sounds large, but for the Sun that’s smaller than most sunspots. So even though the Sun has bright hot regions and dimmer cool regions, they are so small compared to the Sun’s surface there isn’t an overall change in solar luminosity.
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- But the outer layer of Betelgeuse is much less dense than that of the Sun. It is even less dense than Earth’s atmosphere. It’s basically a thin soup of glowing gas. That means the convection regions on Betelgeuse can be huge. A single region can cover a large part of the star. When one of those regions rises to the top, Betelgeuse gets brighter, and when it cools the star dims. Betelgeuse is starting to brighten because hot material is convecting to its surface. This is normal for Betelguese and is likely the way things will be for millennia.
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- New data from NASA’s Hubble Space Telescope shows that its unexpected dimming was most likely caused by its “blowing its top” in 2019, during which it lost a big part of its surface. What scientists are called a “surface mass ejection” (SME) from Betelgeuse is something never before observed.
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- A bright red supergiant star in our galaxy that’s near the end of its life, Betelgeuse likely will explode as a supernova and be visible in the daytime sometime in the next 100,000 years. A supernova hasn’t been seen in our galaxy since the 17th century.
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- Our own star, the Sun, sometimes launches huge clouds of plasma into space called coronal mass ejections (CMEs). At Betelgeuse it’s at an incredible scale with its SME estimated to blast off 400 billion times as much mass.
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- Astronomers don’t often get to watch stellar evolution in real time, normally observing stars in various phases lasting for long time periods. Nor do they often see stars “bouncing,” as Betelgeuse appears to now be doing.
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- By plotting changes in the brightness of the red supergiant star Betelgeuse we witness the titanic mass ejection of a large piece of its visible surface. The escaping material cooled to form a cloud of dust that temporarily made the star look dimmer, as seen from Earth.
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- This unprecedented stellar convulsion disrupted the monster star’s 400-day-long oscillation period that astronomers had measured for more than 200 years. The interior may now be jiggling like a plate of gelatin dessert.
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- Betelgeuse is thought to be recovering from its massive “burp” from within in 2019 when a bubble of gas from deep inside the star produced enough power to blast off a section of its surface, which became a dust cloud above that caused the star to appear to dim as seen from Earth.
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- Betelgeuse is about 750 times the radius of our Sun and 530 light-years away, which is well beyond the 50 light-year “danger zone” for Earth if a nearby star does go supernova.
Of course, Betelgeuse may have already gone supernova over 529 years ago and its light just hasn’t reached us yet.
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September 18, 2022 BETELGEUSE - a star ready to explode? 3685
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