This week marks the 25th anniversary of the launch of the Hubble Space Telescope. This Wednesday night, PBS will be featuring a documentary, Invisible Universe Revealed which will look at the history of this amazing instrument.
I'm looking forward to seeing this documentary, not just because I love the HST, but because a post I wrote in 2007 that mentioned the Hubble caught the attention of those working on the documentary. In particular, I noted that the HST added tremendously to our understanding of stellar formation and evolution and they wanted details. I passed along several thoughts, but I doubt that the hard science will be making the final cut (no, I haven't gotten a sneak peek). So to celebrate Hubble's 25th, here's some of the thoughts I passed along.
Before I start though, I should give my normal caveat that the discovery process is often muddled in modern science and astronomy in particular. One group using one telescope may note something interesting, another using a different instrument does follow up observations, another group does the math, more observations are made by other people, and while it supports the hypothesis, not everyone is convinced and it takes years or decades to form a scientific consensus as more and more results pour in from multiple teams an instruments.
Thus, it is nearly impossible to say "Hubble discovered X". Rarely is astronomy so cut and dry. Rather, we should approach the question from the opposite direction and ask, "What observations might be needed to build and/or support stellar formation theory and has Hubble contributed to any part of that process?"
In particular, there are several things I consider as observational evidence that the theory is correct:
- For a cloud to collapse to form a star in the first place, it will have to surpass what's known as the Jeans Mass (essentially having enough mass in a small enough space with the right conditions). While it's good sound physics, if you really want to confirm the models that rely on this are correct, you'd need to not only demonstrate that the necessary conditions of mass, density, pressure, etc... are being met, but that clouds that meet those conditions are actually collapsing. This can be done via spectroscopy by noting that the edge of a proplyd closest to you is redshifted (i.e., it's collapsing towards the center which is further from you) while the more distant edge is blueshifted (i.e., it's collapsing towards the center which is closer to you). Indeed, Hubble did just this.
- Once a larger nebula has begun to fragment, proto-stars should develop inside the proplyds. Hubble was not the first to observe propylds. In particular, the Infrared Astronomical Satellite (IRAS) launched in 1983, had previously discovered them (for example, here is a paper on them from 1989 although the term "proplyd" had not yet been introduced). However, it was Hubble observations that really did the heavy lifting on proplyds that things seemed to take off from the HST observations. In particular, visual observations from the Hubble seemed to be what determined that these weren't just clumps, but were flattened which is a sign that they're rotating and forming disks as predicted by stellar formation theories. A major paper on this was published in 1994 by O'dell and Wen.
- For stellar formation to work, forming stars will need to find a way to overcome the conservation of angular momentum which requires that as a cloud collapses, it would "spin up" and would result in it flinging itself apart (like a child on a merry go round spinning to fast). Several methods are proposed to do so, but one of the most pronounced is shooting out excess material at high velocities through jets perpendicular to the disk. Such jets have been known since the late 1800's (they're quite large and relatively bright in an astronomical sense since the ejected material slams into the larger interstellar cloud around it at high velocity). The jets themselves are known as Herbig-Haro (HH) objects and at their centers, we often find extremely young stars such as T-Tauri objects. T-Tauri objects had long been recognized as a type of variable star, but again, Hubble seems to have been the first to zoom in on them sufficiently to see their structure. Much like the proplyds, they were discovered prior to the Hubble era, but this 1999 paper suggests that their actual structure hadn't been resolved in detail until the HST. In particular, that paper indicates jets were discovered in some of these objects and points to other papers in which jets were discovered in such objects thanks to the HST.
- Another important clue is that we find young stars in places that we expect them to be forming; namely, in dense dust clouds. The problem is that it's hard to see into these clouds to confirm this. In 2009, the HST got a very nice upgrade with an infrared camera that allowed it to peer through the dust and see these young stars still in the shrouds. Again, this wasn't entirely new. The Spitzer Space Telescope had been launched 6 years earlier, but Hubble was definitely a contributor.
Those are really the main pieces of evidence I'd want to see to be convinced our models of stellar formation were correct. However, there's one more way to look at things: Stellar evolution is a very hard theory to really prove because we don't get to see a star's life from start to finish. Even the births are hidden inside dense nebulae and proplyds and take hundreds of thousands of years. Trying to study this field is like taking a quick hike through a forest and and trying to figure out the entire life cycle of a tree. You can probably do it because you can see saplings to adult trees to rotting logs. But if your hike is too short, you won't have seen enough to really have a coherent picture.
Prior to the Hubble, we'd walked on trails along the edge of the forest, but Hubble took us deep into its heart. It's not always important that we saw new things or saw them for the first time. It's also important that we just saw more of them; enough to really be sure that we had seen all the steps in the process and that it was always consistent. That's not nearly as glamorous, but in science, that's quite often even more important.