Quiz 8

What is a brown dwarf? What makes it different from a planet? Why is it not considered a star?

A brown dwarf is a star-like body with an insufficient amount of mass to cause nuclear reactions in its core and thereby become self-luminous. It has the same characteristics as a star but lacks the mass to trigger hydrogen fusion. They do shine, this means that they are not able to be classified as a planet. (OR) Brown dwarfs are failed stars that didn't have enough mass for sustained fusion. They are more massive than planets, but not massive enough to start fusing hydrogen in their cores.

Two protostars, one 10 times the mass of the Sun and one half the mass of the Sun are born at the same time in a molecular cloud. Which one will be first to reach the main sequence stage, where it is stable and getting energy from fusion? How do you know?

The more massive a star is, the quicker it progresses through each stage of a protostar, as seen in Figure 21.12, Evolutionary Tracks for Contracting Protostars. As a result, the 10 solar mass star, the one that gets its energy from fusion, will first become a true star. (OR) The more massive star will reach the main sequence stage first. More massive protostars collapse faster.

At the end of the p-p chain of nuclear fusion in the Sun, hydrogen nuclei have been converted into:

a helium nucleus

When a star settles down to a stable existence as a main-sequence star, what characteristics determines where on the main sequence in an H-R diagram the star will fall?

its mass

Astronomers believe that disks of material will form around protostars that are spinning. Which of the following observed phenomena is a good indication of the presence of a disk around a protostar?

jets and Herbig-Haro objects

When two light elements collide to undergo nuclear fusion,

some of the energy in their mass is released

What observations about disks of dusty material around young stars suggest that planets may be forming in such disks?

the disks show lanes that are empty of dust within them

A college friend of yours who has been postponing taking any science courses hears you talking about the generation of nuclear energy in the Sun and makes the following observation: "The whole idea of the atomic nucleus is pretty ridiculous. If an oxygen nucleus consists of eight protons and eight neutrons, the charge on that nucleus is positive. Since even I learned in high school that like charges repel, such a nucleus would find all its positive protons repelling and quickly fall apart." How would you answer his argument?

the nuclear force, which is attractive over short distances like the nucleus, and stronger than electricity, holds the nucleus together

If it takes an average of 14 billion years before any proton inside the Sun will undergo fusion, and the Sun is only about 5 billion years old, why do astronomers believe that fusion is going on there now?

there are an enormous number of protons inside the Sun, and some of them will fuse much sooner than the average

A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram?

to the left

Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star?

when nuclear fusion reactions begin inside its core

A Herbig-Haro (HH) object is

where a jet from a star in the process of being born collides with (and lights up) a nearby cloud of interstellar matter