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:
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):
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:
Psc-Eri stream also seems to contain: brown dwarf candidate HD 1160B (now at Pleiades age -> it's a 0.12 Msun star!), bright eclipsing binary Lambda Tauri, and some other naked eye stars: Omicron Aqr, Nu For, 106+108 Aqr, Tau1 Aqr, 26 Psc, Upsilon Gru(mostly ~80-226 parsecs away)
— Eric Mamajek (@EricMamajek) May 28, 2019
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.
Wow! Coeval population of 2000 stars only 100pc away and 1 Gyr old!
This will be another important datum for studies of stellar evolution.
— Jason Wright (@Astro_Wright) January 23, 2019
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:
I've never liked "moving group" as "moving" always seemed redundant. "Association": usually see the clump of stars, tend to be young. "stream" is probably more appropriate as it was found in velocity space, wasn't obvious spatially until selected by velocity.
— Eric Mamajek (@EricMamajek) January 24, 2019
Jason, Eric, and I then took the conversation offline. First, it needed a name. Eric came up with the “right” answer:
@EricMamajek named the stream Pisces-Eridanus (Psc-Eri) in the OB association tradition, as it appears to be an older version of such groups (Sco-Cen). Many members are in Psc & Eri, along with its convergent point. I also love the fish/river symbolism for this stellar stream.
— Jason Curtis (@jasonleecurtis_) May 28, 2019
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:
"TESS reveals that the nearby Pisces-Eridanus stellar stream is only 120 Myr old": my latest paper with Marcel Agüeros, @EricMamajek, @Astro_Wright, and Jeff Cummings: https://t.co/2jzOdWRXh4 https://t.co/1M5B7Ej4mh
Here, you see distance vs age for clusters w. rotation periods. pic.twitter.com/7ydEtLtJb7
— Jason Curtis (@jasonleecurtis_) May 28, 2019
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:
It’s funny that this is getting so much attention now that it is only 120 Myr old. I was rooting for it to be 1 Gyr or older, as originally thought. IMO that would have been even more interesting!https://t.co/qDKuPguVFu
— Jason Wright (@Astro_Wright) May 29, 2019
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:
I hope it’s ok that I hate this framing. Great result, but it’s not a new Pleiades in any sense that will mean anything to most observers
— chrislintott (@chrislintott) May 28, 2019
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:
‘Framing’: Scientifically, it will be a *great* control sample for the Pleiades. Pleiades has dominated our understanding of how ~100 million year old stars and brown dwarfs behave (rotation, lithium, activity, etc), but not clear if typical (most clusters disperse in ~10 myr). https://t.co/vrJ4Ced9GF
— Eric Mamajek (@EricMamajek) May 29, 2019
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:
* @jasonleecurtis_ points out actually making the plots themselves only takes *hours*. Months is the careful vetting and paper writing and referees reports and…
— Jason Wright (@Astro_Wright) May 17, 2019
It’s a new era of stellar astronomy. So exciting!