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Stargazing 4

Title text: We haven't actually seen a star fall in since we invented telescopes, but I have a list of ones I'm really hoping are next.

Explanation

This is the fourth comic in the Stargazing series, and it followed 2274: Stargazing 3 that came out five years before. That was the longest stretch between two comics in the series so far.

The host Megan begins the introduction by referencing rude Yelp reviews of her stargazing lessons. The reviewers doubt that she is actually a qualified astronomer due to how simplistic her lessons are; they claim she is just saying the words that come to mind.

Then she states that there are over 20 stars in the sky and some of them are over the age of 100. Both of these statements are true, but extreme understatements.

• A few thousand stars are visible to the unaided eye under good viewing conditions, though in a city there could be less than 20 stars visible even on a clear night.

For a normal stargazing session the event should be held in a venue with as little light pollution as possible, which could mean the middle of an urban green space, conveniently away from lighting or else specially arranging for the most inconvenient lighting to be off for the duration.

However, given the unprofessional nature of Megan's lessons, there is no guarantee that this session does not take place under less than ideal circumstances. Or she is perpetually unlucky as daylight or clouds may further reduce visible stars.

Ignoring the need for visibility entirely, it is also estimated that there are about 200 sextillion (i.e. 2x10²³) stars in the universe. Of which around a half would be somewhere 'in the sky' at any given moment.

• Stars are typically billions of years old. While new stars are being created in nebulae all the time, it is extremely unlikely that we are seeing the nebulous start of even the shortest lived stars within the first century of their life.

One of the 'youngest' potential candidates is SN 1987A, which may be a neutron star of less than 40 years old. But that is discounting the additional age it has acquired from it being approximately 168,000 light years away from us (making it actually 168,038(ish) years old, already). It is further undermined by arguably just being the next stage of life of the far older star that had to go supernova in order to leave it behind.

Of stars within 100 lightyears of Earth, and formed afresh from interstellar material, AU Microscopii is slightly over 30 lightyears away and considered to be very new (as far as stars go). But it is still 22 million years old, as it is currently understood.

Megan states that our galaxy is huge and that there are more grains of sand in the Milky Way than grains of sand on all of Earth's beaches. This is a parody of the common saying that there are more stars in the visible universe than grains of sand on all the beaches of Earth. Since the Earth's sand is a subset of all of the galaxy's sand, and there are more planets with sand other than Earth (such as Mars), there are unquestionably more grains of sand in the Milky Way than on Earth. Tangentially, it is unclear whether the stars outnumber Earth's sands, as shown here: Do Stars Outnumber the Sands of Earth’s Beaches? and here: The ever-lasting question: more sand or stars?. Also, the original quote was all the sand on Earth, not just on the beaches. Megan adds a helpful hint, calling a beach a big wet sandbox.

She then finishes the lesson by correctly saying that there is a black hole in the center of our galaxy (Sagittarius A*), and that stars sometimes fall in and get consumed by the black hole. When stars come too close to black holes, they experience a tidal disruption event (TDE), where a star is pulled apart by the black hole after exceeding its Roche limit. This creates streams of material that orbits the black hole and forms an accretion disk, that will eventually be consumed by the black hole or ejected in jets.

She adds her personal opinion on this fact saying that such events are "hilarious" and proceeds by saying that it's okay to laugh at the fate of those stars as the gravity of the black hole will prevent any signals from those stars escaping. This is due to black holes' immense gravitational attraction that prevents even light from escaping. In Megan's case the most important consequence of this fact is that anyone on planets around such stars cannot leave Yelp reviews if they hear her laughing. Thus, they cannot add to those that mock her lesson.

However, as the Roche limit of a black hole is always greater than its Schwarzschild radius, reviews made just after the star begins spaghettification could still escape the black hole. Not only do stars not use any kind of human-made technology,^{[citation needed]} but any information regarding the app Yelp has yet to reach any star near Sagittarius A*, and will only reach it in 27 thousand years. It is much more likely that someone living on one of the star's planets would try to leave a comment on Yelp, not the star itself. But the same issues with distances would of course apply. It also seems unlikely that any planet would still be following a star when it first gets that close to a super massive black hole.

In the title text Megan claims that we haven't actually seen a star fall into a black hole (or the black hole that she is describing) since we invented telescopes. In reality, over a hundred TDEs have been discovered in a variety of wavelengths, including by the Hubble telescope. The way title text is phrased though she might have meant fall into specifically Sagittarius A*, and while it is true that we haven't observed any star fall into our closest supermassive black hole, the G2 gas cloud on an accretion course was discovered in 2002. Megan also apparently has a list of stars she would like to see fall into the black hole. But she can keep hoping as humans at this time have no way of changing the position of any star, and probably couldn't implement it soon upon such distant stars, even if were possible. So unless she is hoping for one (or more) of the already closer stars to be observed to fall in next, she is unlikely to experience success based on her list.

Transcript

[The background is black, Megan is in front of three others, Cueball and Ponytail to the left, and White Hat to the right. Megan is drawn in white while the background characters are in grey.]

Megan: Welcome back to Stargazing.

Megan: According to some incredibly rude Yelp reviews, I'm "not informative" and "can't possibly be an astronomer" and "just kind of say words as they occur to me."

Megan: I guess some losers just hate cool space facts!

[The sky is white, the floor is black, Megan is pointing at the sky. Hairy, a Megan-like woman (with a longer hair), Cueball, and Ponytail are on the left, White Hat and Hairbun are on the right. All of the characters are drawn in black.]

Megan: Anyway, that dot is a "star."

Megan: There are over 20 stars in the sky, and some of them are more than 100 years old.

[The background is black again, the panel zooms in on Megan's face.]

Megan: Our galaxy is huge.

Megan: You know the beach? That big wet sandbox?

Megan: Well, there are more grains of sand in the Milky Way than in all Earth's beaches combined.

Off-panel voice to the left: Wow.

Off-panel voice to the right: ...Wait.

[The background is still in black, Megan is in front of others and has her finger raised, Ponytail is left of Megan, White Hat and Hairbun on the right. Megan is again drawn in white while the background characters are in grey.]

Megan: The galaxy has a black hole at the center.

Megan: Sometimes stars fall in, which is hilarious.

Megan: Don't worry, it's okay to laugh. The gravity prevents signals from escaping, so they can't leave Yelp reviews.

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