A Second Pleiades in the Sky

Astronomers have discovered a second Pleiades in the sky.

Wait, what?  The Pleiades are an obvious feature of the night sky, known to ancient peoples around the world and a common test of visual acuity.  In a telescope they look like this:

The Pleiades

Four or five bright stars make them obvious in even a moderately dark sky, and the nebulosity adds a nice touch that makes them interesting. Astronomers like them because they were all formed from the same birth cloud, so they share composition, age, and distance. This makes for a great stellar laboratory, since it lets us isolate and focus on the few differences among the stars, like their mass and whether they have binary companions.

So if these are so obvious, how could there be another one?  Answer: if they’re so spread out across the sky that no one noticed they were related!  Back in February Meingast et al. announced that they had spotted a group of hundreds of stars all moving in the same direction—but they were spread all out against the sky.  If you plot most of the sky out flat, here’s what they look like (in red):

The Pisces-Eridanus Stream. The Milky Way, which spans 360 degrees of the sky, appears as a circle in this projection. The stream is over 120 degrees across.

Co-moving stars like this are like clusters but more spread out. It took the precise measurements recently announced by the Gaia team to notice that these stars were all moving in the same direction.

Many of these stars, like the Pleiades are bright enough to see with the naked eye:

When Meingast et al. published, I got excited because they guessed that the stream was 1 billion years old—if correct, that would make it one of the closest older clusters in the sky. Jason Curtis had spent a whole PhD dissertation and more proving Ruprecht 147 was a true cluster, 3 billions years old and only 300 parsecs away. Eunkyu Han worked hard to check out another claimed nearby old cluster, Lodén 1.  Here was one 3 times younger and 3 times closer: another important discovery! I got excited and tweeted at Jason about it.

We started wondering what to call it, and also getting suspicious of the reported age. So we pulled in the expert on “moving groups” and stellar ages, Eric Mamajek:

Jason, Eric, and I then took the conversation offline. First, it needed a name.  Eric came up with the “right” answer:

Next, the stream seemed like it had to be younger than 1 billion years to us, but how old was it?  Then Jason Curtis went to town with TESS, the all-sky planet hunting telescope. It had already hunted for planets around many of the stream’s stars, and Jason was able to quickly measure the stars’ rotation periods.  He tells the story here:

I encourage you to read the whole tweet thread!

Basically, Jason was able to show that the stars in the stream are spinning way too fast to be 1 billion years old. In fact they were spinning just as fast as the Pleiades—so they are probably almost exactly the same age.  They’re also the same distance, and there are just as many of them!

I was actually kind of disappointed:

Chris Lintott was grumpy at my framing of the cluster as a “second Pleiades” because the public would understand that to mean “another thing I can see with my eyes in the sky that wasn’t there before”, which is misleading:

His point is well taken, but Heidi Hammel would say that good science communication means linking to things people know about.  Eric explained well what I meant and why the discovery is a big deal:

This discovery came very fast, and was only possible because of the hard work of the teams that made Gaia, Kepler, and TESS possible.  Because those are all-sky surveys committed to making data public as fast as possible, unexpected gems like this can go from tweets to papers in a matter of weeks.  Jason Curtis pointed out that most of the actual science only took him hours:

It’s a new era of stellar astronomy. So exciting!

One thought on “A Second Pleiades in the Sky

  1. Harry R Ray

    My comment has absolutely nothing to do with the Pisces-Eradanus Stream(sorry if that is a problem)but instead, with a potential “framing issue” caused by a recent preprint, ArXiv:1905.11419. “The Chaotic Nature of TRAPPIST=1 Planetary Spin Rates,” by Vinson A., Tamayo D. & Hansen B.(Jason: If you have not already read the PDF, please do so! The results are fascenating and a bit controvercial if they hold up to scrutiny and are published in a respected scientific journal). Now, for the possible “framing issue”: What to name this phenominon if it proves to be true? In my own opinion(which may or may not be correct)it should be called an “effect”. Should this be the proper term, the logical conclusion would be to call it “The Vinson-Tamayo-Hansen Effect”. However, since TRAPPIST-1 is a very popular topic with the general public right now, I cede to your “Second Pleides” nomenclature and suggest a far more understandable (to the general public)name, which is sure to garner much more attention: “The Westeros Effect”. Do you think that this would be appropriate?

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