Examples of SEDs
As we discussed in an early blog post, the SED (Spectral Energy Distribution) is a plot of how bright an object is are as a function of wavelength. You can find each object’s SED on its Talk page.
Tadeáš Černohous (TED91) put together a wonderful collection of some of the different kinds of SEDs that you’re likely to encounter on Disk Detective. Nice work, TED91! Here it is for your delight. I added a few comments here and there. –Marc
Early Type Stars (Debris Disks)
These stars are as hot as the Sun or even hotter, and generally have spectral types B, A, F or G. (Type O is even earlier than that, but these very massive stars are rare.) The SEDs for these stars are nearly straight, downward-sloping, lines because of the Rayleigh-Jeans law. You’ll notice however that the very last point always falls slightly above the straight line; that’s because all the objects in Disk Detective were pre-selected for this property. This “infrared excess” is a sign that they might be surrounded by a disk, which glows only at these longer wavelengths.
Late Type Stars (Debris Disks)
These are stars of spectral type K or M, which are cooler and don’t emit as much at 1 micron as a the early type stars. You’ll see that the first point of the SED is a bit lower for these objects. The physics of this phenomenon is the Wein Displacement law; the peak of the blackbody curve shifts to longer wavelenegths for cooler objects.
Another effect that can cause the first few points of the SED to drop is interstellar “extinction” or more specifically, interstellar “reddening”. That’s when interstellar dust between the star and the Earth absorbs some of the light at the short wavelength end of the SED.
Young Stellar Objects (YSOs)
YSOs generally have more infrared excess than debris disks, and the excess kicks in at shorter wavelengths, even as short as K band. Many YSOs are also reddened by interstellar dust. The SED’s of these objects sometimes may look similar to those of Active Galactic Nuclei (AGN) and dusty red giant stars; it’s hard to distinguish among them. If you aren’t sure which one it is, try to find some information on SIMBAD or VizieR.
See that point at 3 microns (fourth point from the left)? It’s “lower” than it should be. That sometimes happens when the light from a bright object “saturates” the detectors in the WISE 1 band. Most often, when you see this in the SED, you’ll also see that the WISE 1 image looks very misshapen or displaced from the crosshairs. These objects are NOT good candidates.
The spectral distributions for galaxies can contain several components: stars of different types as well as dust at various temperatures. Moreover, galaxies can be redshifted by the expansion of the universe, a process that shifts the SED to the right, sometimes even halfway across the plot. We aren’t interested in these objects in this phase 1 of Disk Detective, though we might be in the future.
Quasars and Active Galactic Nuclei (QSO and AGN)
These objects may look like stars to you at first glance, because they often appear as a point sources of light. But these SEDs are clearly very different from those of stars. This is one of the ways SEDs can be useful! Quasars and AGN are also classes of objects we aim to discard in phase 1 of Disk Detective.
Planetary nebulae have nothing to do with planets. They are clouds of gas and dust belched out by an old red giant star. These objects are fascinating–but for the purposes of Disk Detective they are trash.
Please note that all of these are just some common examples. The SEDs may be different from case to case, especially when those objects are somehow contaminated or blended.