March Night Sky
Sky events for this month & objects for observation can be found here.
Courtesy of Orion Telescopes & Binoculars
Click to enlarge. A larger printable version of this chart can also be found here.
ORION NOT QUITE THE SAME
The circled-dot star chart symbol for Betelgeuse indicates Alpha Orionis is a "variable star" with known changes in brightness.
The constellation Orion is one of the most widely known and beloved in the sky, and most prominent during the long northern hemisphere winter evenings.
Alpha Orionis – the shoulder of the constellation Orion the Hunter – is one of the easiest to recognize stars in the night sky, and has a distinctively orange-red tint. Commonly known as Betelgeuse – it’s also called a Red Supergiant and one of the largest stars we know, with a radius which would extend out beyond the distance of Mars’ orbit from our Sun. In recent months, this bright star has become noticeably fainter.
UPDATE: The latest observations of Betelgeuse show that the star is now beginning to slowly brighten:
AAVSO / James Bedient
The changing brightness of Betelgeuse is most likely 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. 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 about 1000 km in diameter. 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.
But the outer layers of Betelgeuse are much less dense than that of the Sun. It is even less dense than Earth’s atmosphere. This 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 likely because an unusually large cloud of gas has begun to dissipate.
But the outer layers of Betelgeuse are much less dense than that of the Sun. It is even less dense than Earth’s atmosphere. This 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 likely because an unusually large cloud of gas has begun to dissipate.
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An artist’s impression of Betelgeuse, showing it surface as enormous convective clouds, and the scale of its size and the surrounding dust clouds. AU stands for "Astronomical Unit," which is the distance from the Sun to the Earth. The size of the Sun and its planets are greatly exaggerated. ESO/L. Calçada
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Brightness curve of Betelgeuse over the past 35 years. AAVSO
In a recent (January 29th) Astronomer's Telegram (#13410) concerning the star, astronomer Edward Guinan of Villanova University writes that during his 25 years of photometry, "Betelgeuse is currently the coolest and least luminous yet observed." Betelgeuse is now around 36 percent of its usual brightness, according to an European Southern Observatory press release. All red supergiant stars will end their lives in a supernova, which is when a star collapses in on itself after it has burned through all of its lighter elements and builds up a core of iron.
This comparison image shows the star Betelgeuse before and after its unprecedented dimming. The observations, 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.
ESO/M. Montargès et al.
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This image, obtained with the VISIR instrument on ESO’s Very Large Telescope, shows the infrared light being emitted by the dust surrounding Betelgeuse in December 2019. The clouds of dust, which resemble flames in this dramatic image, are formed when the star sheds its material back into space. The black disc obscures the star's centre and much of its surroundings, which are very bright and must be masked to allow the fainter dust plumes to be seen. The orange dot in the middle is the SPHERE image of Betelgeuse’s surface, which has a size close to that of Jupiter’s orbit.
ESO/P. Kervella/M. Montargès et al., Acknowledgement: Eric Pantin
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Could this be a sign that Betelgeuse is about to explode as a supernova? Astronomers say probably not. Betelgeuse is a well-known semiregular variable star, and its changes in brightness have been monitored for years by amateur and professional astronomers working with the American Association of Variable Star Observers (AAVSO). We therefore know that there are multiple cycles for Betelgeuse’s rising and falling brightness. When the minimums of all the cycles become momentarily synchronized the star could look unusually faint, as it currently does. But the fact remains Betelgeuse is now dimmer than it has been anytime in the recent past.
Could this be a sign that Betelgeuse is about to explode as a supernova? Astronomers say probably not. Betelgeuse is a well-known semiregular variable star, and its changes in brightness have been monitored for years by amateur and professional astronomers working with the American Association of Variable Star Observers (AAVSO). We therefore know that there are multiple cycles for Betelgeuse’s rising and falling brightness. When the minimums of all the cycles become momentarily synchronized the star could look unusually faint, as it currently does. But the fact remains Betelgeuse is now dimmer than it has been anytime in the recent past.
As seen in the left picture, Betelgeuse (arrow right pane) usually rivals Rigel (arrow left pane). The right picture on Dec 22, 2019 by Bob King shows how unusually dim Betelgeuse has become.
And of course in this day and age things often are sensationalized, which has caused some speculation that Betelgeuse could be about to go supernova. Astronomers say this is unlikely any time in the next 100,000 years. What are some other possibilities for Betelgeuse’s strange and dramatic dip in brightness? Astronomers suggest that the change in brightness could be due to the changes in the layers of gases expelled by and surrounding the star.
What would happen if Betelgeuse were to explode? While Betelgeuse isn’t among our closest stellar neighbors, it is normally one of the brightest stars in Earth’s sky. The reason is that Betelgeuse is a supergiant star, and it is by nature very brilliant – 90,000 to 150,000 times more luminous than the Sun. Such luminosity comes at a price. Betelgeuse’s tremendous energy output requires that its fuel be expended relatively quickly, and in fact Betelgeuse is now near the end of its lifetime. Sometime in the future it will essentially run out of fuel, collapse under its own weight, and then rebound in a spectacular supernova explosion. When this happens, Betelgeuse will brighten immensely for a few weeks or months, perhaps to be as bright as the full moon and even visible in broad daylight. While Betelgeuse is “nearby” in relative terms, it’s still some 720 light-years from Earth, and there would be little if any direct effect on the Earth.
So while Betelgeuse has become noticeably fainter in recent months, does that mean it’s about to explode? Probably not… But it is nice to speculate about what that would actually look like!
