VY Canis Majoris mass-loss history sheds light on hypergiant's state, connection to Betelgeuse - NASASpaceFlight.com - NASASpaceflight.com
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In the southern hemisphere, in the constellation Canis Major, lies a very young, very massive red hypergiant star called VY Canis Majoris (VY CMa)

A pulsating variable, its apparent magnitude (how bright it appears in the night sky) varies unpredictably.  It is located 1.2 kiloparsecs, or approximately 3,900 light years, from Earth, is only 8.2 million years old, and is one of the largest stars known to exist

It is also one of the brightest stars known in the Milky Way galaxy, being 350,000 times more luminous than the Sun.  Yet for the last 220+ years, we have not been able to see it with the naked eye since a dimming event in the late-1800s that the star has never visually recovered from

The first (surviving) observation in 1801 gave the star an apparent magnitude just at or near +6.5 — barely (depending on atmospheric conditions) visible to the naked eye.  This was calculated by eye, and photographic plates to confirm the apparent magnitude did not exist at the time

Other recorded observations from 1830 and a cluster around 1848-1850 returned similar +6.5 apparent magnitude observations — with even a few estimates closer to +6, indicating a potential, slight brightening of the star

A 20 year gap into the early 1870s then exists.  But it’s in the 1870s data that things really start to get interesting.  Huge swings in apparent magnitude appear, rapid fluctuations from +6.5 down to +8 (too faint to be seen with the naked eye)

These wide swings continued throughout the 1870s.  But after each major dimming event, VY Canis Majoris returned more or less to an apparent magnitude of +6.5

✔️ MYSTERY SOLVED: Like red supergiant star Betelgeuse, astronomers sought to explain why red hypergiant VY Canis Majoris varied in brightness

Observations in the 1880s showed a potentially steady apparent magnitude of +7.3, but records are sparse.  By the 1890s, near continuous observation of the star began.  And two big things stand out in the data:

So what has caused all these dimming events?  What happened in the 1880s that caused the star to not recover as it has from every other dimming event?  And don’t these dimming and brightening events sound familiar in terms of Betelgeuse’s dimming surprise of 2019/2020

But first, to understand the observed dimming events of VY Canis Majoris, we must look at the clumps, knots, and arcs of ejecta material at the heart of Humphreys et al.’s recent work

As seen in the above image, the inner knot, which is estimated to have been ejected from VY CMa 30-35 years ago (light time received at Earth) corresponds to the 1987 and 1992 dimming events.  Likewise, the S knot B and W1 knot C’s calculated ejection times overlap with each other and correspond to the 1870s dimming events (and are likely linked in terms of time of ejection to the major event from which VY CMa has not visually recovered in apparent magnitude)

As the ejection events begin, a large cloud of material is violently thrown from the star — blocking some of its light from reaching Earth.  The greater the event in terms of material, the more significant the dimming event appears

Artist’s impression of hypergiant star VY Canis Majoris with its vast convection cells and violent ejections

The speed at which the material travels away from the star, calculated to be between 20-30 km/s−1, causes the material to quickly move out of the direct line of sight to VY CMa — thus explaining why the star dims and then recovers to its previous apparent magnitude

So then what happened in the 1880s that caused the star to not recover as it has from every other dimming event

noted in their paper, “The most interesting period may be 1870-1880 and the post-1880 fading from which VY CMa has not recovered.  S knot B and W1 knot C plus possibly the W2 knot have ejection ages that correspond to this period.  Their different locations and orientations with respect to the star suggest that surface activity occurred over much of the star with separate outflows in different directions lasting at least 10 years.  The major dimming of the star after 1880 by one or more magnitudes is very likely the origin of the present obscuration of the central star.”

So then how does this all relate to Betelgeuse?  Much attention was given to the star from October 2019 to February 2020 when the usually reliable, semiregular variable star (which usually fluctuates in apparent magnitude from just +0 and +0.5 over predictable 425 day and 5.9 year cycles) suddenly, rapidly, and unexpectedly dimmed to a minimum of +1.614

Thus, what is happening with VY Canis Majoris and its large-scale ejection events can shed light on what is happening with Betelgeuse.  And in fact, a recent paper published by Dupre et al., focusing on the Betelgeuse dimming event, concluded that a major ejection event occurred on the star — obstructing much of the light from the star’s southern hemisphere until it traveled out of our line of sight, causing an increase in brightness

And this would make sense given that both Betelgeuse and VY Canis Majoris are predicted to go supernova within 100,000 years.  While VY Canis Majoris is more active in its mass-loss events than Betelgeuse has now been observed to have gone through, this type of behavior from red hypergiants and red supergiants are common

Both stars are well known for various reasons, and both will provide spectacular end-of-life light shows on Earth when they go supernova.  It’s possible that what is being seen with VY Canis Majoris right now is the star proceeding through the final phase of its evolution before the terminal period begins.  If so, it could provide clues on what to expect from Betelgeuse as it too races toward the end of its brief celestial existence

But these high mass-loss events from two nearby giants do not simply serve as end of life indicators or explanations for VY Canis Majoris and Betelgeuse’s dimming events — they are life-giving eruptive forces to the universe as well

(Lead image: VY Canis Majoris obscured by is nebular gasses and material (left); Betelgeuse during its dimming event in February 2020 (right). Credit: HST, NASA, ESA, ESO, Humphreys et al. Edited by Brady Kenniston)


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