Why does the professor say this:

Hello, class. I hope everyone’s good today, um, I’ll be covering an extremely interesting aspect of star formation today--urn, actually, it’s the reverse aspect of it. Umm, I want to give you a glimpse into the degeneration of stars, their slow cosmic death so to speak, yeah? Wonderful. So to that purpose I’ll focus my discussion on white dwarfs and black dwarfs, and what sets them apart from the rest of interstellar objects.
    Um, right, so let’s proceed by watching this splendid image taken from space by the Hubble telescope. Now, see that pulsating white light? Yeah? Well, that’s Sirius--ah, the brightest star in our universe. And right next to it, there, to its right, did you catch that? It’s a faint luminous dot. That’s its companion star, called Sirius B. It was discovered in 1862 by astronomer Alvan Graham Clark, and it’s the first observed instance of a white dwarf.
    OK. So uh, now what sets these white dwarfs aside from the rest of the stellar universe? Well, it seems they are a form of degenerate stars, an instance of stars that are dying out. How do we know that? First, because they are so much dimmer than more mature bright stars. Urn, and second because they are also much tinier in cosmic terms, some are even smaller than planets. Like Sirius B, for instance, it’s barley as large as our planet Earth--and that’s very small when you think of Jupiter, or Saturn, or the like.
    What makes these white dwarfs different though is their mass. Sirius B, for example, packs half the density of our Sun into its little frame. That makes white dwarfs the heaviest forms of matter in the universe apart from black holes and neutron stars.     Hmm, so what does that mean for this celestial body? Urn, how was it possible it acquired so much mass in spite of its reduced volume? White dwarfs are the last phase in the existence of a star. So, when a star has spent all of its hydrogen to fuse into helium, it goes through a red giant phase, alright? That means it gets to shed its outer layer to create a planetary nebula--everybody familiar with that term? Perfect! So this expelling of its surface, um, that takes up lots of energy, right? The star has no more hydrogen left to sustain nuclear fusion--you know, it’s basically an inert core made up of carbon and oxygen, that’s it, OK? Since it’s no longer emitting energy, it shows up like a blur on the night sky--and that’s our white dwarf.
    Now, what happens to white dwarfs is that they start to cool down --seeing as there’s no energy to sustain them. This gradual shift in temperature causes the core mass to shrink within itself--so that’s why white dwarfs have such abnormally high densities. Makes sense, doesn’t it? What usually happens now is that density pressure causes gravitational collapse-- or, in other words-- a giant explosion of the supernova type. I gather you all witnessed the one shown on TV just recently, right?
    But of course, most white dwarfs don’t meet such violent ends. Actually, most of them gradually fade and burn off until they disappear completely. In this last stage, they are actually called black dwarfs because they emit so little radiation that they can’t even be detected by current astronomic devices.
    Truth be told, no black dwarf has yet been discovered in the universe partly because the universe is still too young to produce them, and partly because even if they did exist-they’d get lost in the cosmic microwave background radiation. Um, careful here though, let’s not confuse black dwarfs with brown dwarfs. Brown dwarfs are also really faint as they can’t sustain nuclear fusion either, but that happens because they don’t have enough mass to do so-so that is opposite to what happens in the terminal phase of heavy white and black dwarfs, OK? Brown dwarfs form through gas accretion and contraction-they are not degenerate stars, quite unlike that actually.
    P Hmm, so what does that mean for this celestial body? Um, how was it possible it acquired so much mass in spite of its reduced volume? White dwarfs are the last phase in the existence of a star. So, when a star has spent all of its hydrogen to fuse into helium; it goes through a red giant phase, alright? That means it gets to shed its outer layer to create a planetary nebula--everybody familiar with that term? Perfect!
    What does the professor mean when he says this:
    P ... everybody familiar with that term? Perfect!
    P Um, careful here though, let’s not confuse black dwarfs with brown dwarfs. Brown dwarfs are also really faint as they can’t sustain nuclear fusion either, but that happens because they don’t have enough mass to do so --so that is opposite to what happens in the terminal phase of heavy white and black dwarfs, OK?
    Why does the professor say this:
    P Um, careful here though, let’s not confuse black dwarfs with brown dwarfs.