Astronomy Chapter 4

Quantinization

The fact that light and matter on small scales behave in a discontinuous manner, and manifest themselves in the form of tiny "packets" of energy, called quanta.

(b) has more protons than electrons

An atom that has been ionized (a) has equal numbers of protons and electrons; (b) has more protons than electrons; (c) is radioactive; (d) is electrically neutral

spectroscope

An instrument that separates light into a spectrum.

- Composition -Motion - Temperature

Astronomers analyze starlight to determine a star's

fewer exited states

Compared with a complex atom like neon, a simple atom such as hydrogen has

(b) broad and fuzzy

Compared with slowly rotating stars, the fastest spinning stars have absorption lines that are: (a) thin and distinct; (b) broad and fuzzy; (c) identical to the lines in the slowly rotating stars

absorption line

Dark line in an otherwise continuous bright spectrum, where light within one narrow frequency range has been removed.

Atoms Key Points

Electrons in atoms can occupy only certain orbits, with precisely defined energies; the allowed energies increase as we move outward from the nucleus. When an electron moves from one orbit to another, the law of conservation of energy says that the change must be accompanied by the emission (if the electron drops from a high-energy orbit to a lower one) or absorption (low to high) of a photon of radiation. The energy of the photon is exactly equal to the change in orbital energy. High-energy photons have a shorter wavelength (higher frequency).

(d) no significant change becuase the absorbption lines are the like a barcode and do not change.

Figure 4.3 in the textbook ("Elemental Emission") shows the emission spectrum of neon gas. If the temperature of the gas was increased, we would observe

photons

Individual packet of electromagnetic energy that makes up electromagnetic radiation.

ionized

State of an atom or molecule that has lost one or more of its electrons.

Star C is moving away from us faster than star B, star A is moving towards us.

Suppose a particular spectral line has a wavelength of 500 nm in the lab. If you look at the spectrum of star A, you see this line at 480 nm. Star B exhibits this line at 510 nm, and the line is measured to be at 530 nm in star C. Which of the following statements is true?

Why are gamma rays dangerous

The basic reason that gamma rays are so much more dangerous to life than visible light is that each gamma-ray photon typically carries millions, if not billions, of times more energy than a photon of visible radiation.

quantum mechanics

The laws of physics as they apply on atomic scales.

Which of the following blackbody curves indicates the coldest object?

c

photon energy∝radiation frequency.

the energy carried by a photon had to be proportional to the frequency of the radiation

photoelectric effect

Emission of an electron from a surface when a photon of electromagnetic radiation is absorbed.

Bohr model

First theory of the hydrogen atom to explain the observed spectral lines. This model rests on three ideas: that there is a state of lowest energy for the electron, that there is a maximum energy beyond which the electron is no longer bound to the nucleus, and that within these two energies the electron can only exist in certain energy levels 1. There is a state of lowest energy—the ground state— which represents the "normal" condition of the electron as it orbits the nucleus. 2. There is a maximum energy that the electron can have and still be part of the atom. Once the electron acquires more than that maximum energy, it is no longer bound to the nucleus, and the atom is said to be ionized; an atom missing one or more of its electrons is called an ion. 3. Most important (and also least intuitive), between those two energy levels, the electron can exist only in certain well-defined energy states, often referred to as orbitals.

fluorescence

A process in which phosphorescent material converts radiation into visible light

molecule

A tightly bound collection of atoms held together by the atoms' electromagnetic fields. Molecules, like atoms, emit and absorb photons at specific wavelengths.

emission line

Bright line in a specific location of the spectrum of radiating material, corresponding to emission of light at a certain frequency. A heated gas in a glass container produces emission lines in its spectrum.

atoms

Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus surrounded by negatively charged electrons.

slightly fewer absorption lines

Compared with a spectrum from a ground-based observation, the spectrum of a star observed from above Earth's atmosphere would show

lower energy

Compared with an electron transition from the first excited state to the ground state, a transition from the third excited state to the second excited state emits a photon of

emission spectrum

The pattern of spectral emission lines produced by an element. Each element has its own unique emission spectrum.

line broadening

can be due to a variety of causes - not the result of some inadequacy of our experimental apparatus; rather, it is caused by the environment in which the emission or absorption occurs—the physical state of the gas or star in which the line is formed caused thermal motion within the gas, emission and absorption lines are observed at frequencies slightly different from those we would expect if all atoms in the cloud were motionless.

Electromagnetic frequencies

electron transitions within molecules produce visible and ultraviolet spectral lines (the largest energy changes). changes in molecular *vibration* produce infrared spectral lines. changes in molecular *rotation* produce *spectral lines in the radio part of the electromagnetic spectrum *(the smallest energy changes). --- FIGURE 4.13 Molecular Emission The carbon monoxide (CO) molecule undergoing (a) a change in which an electron in the outermost orbital of the oxygen atom drops to a lower energy state (emitting a photon of shortest wavelength, in the visible or ultraviolet range), (b) a change in vibrational state (of intermediate wavelength, in the infrared), and (c) a change in rotational state (of longest wavelength, in the radio range).

How are absorption and emission lines produced in a stellar spectrum? What information might absorption lines in the spectrum of a star reveal about a cloud of cool gas lying between us and the star?

Absorption lines are produced when a certain element or molecule is hit with another light and absorbs that light, the absorption spectra shows black lines where no light gets through to the element. When the electron jumps back up it must absorb that same frequency photon On the other hand, in the emission spectra, colored lines are shown when an element produces or emits that certain color as an electron jumps back down to their energy level, according to their specific frequency, they will emit the corresponding color photon. The absorbption lines

Zeeman effect

magnetic fields can also broaden spectral lines

third state, second, first, ground

When electrons are excited to different energy levels, the average radii from the nucleus also changes. Rank the following electron energy states according to the average distance of the electron from the nucleus.

No astronomical object that produces a continuous visible spectrum of light has ever been observed. However, there are many astronomical objects that produce emission or absorption spectra. Read the following descriptions of astronomical objects, and then sort the labeled images into the appropriate bins according to the type of spectrum each object produces.

- Emission nebula: a cloud of hot, interstellar gas glowing as a result of one or more nearby young stars that ionize the gas. - Planetary nebula: a glowing cloud of hot, low-density gas that is ejected from a red-giant star. - Sun: a glowing ball of extremely dense gas powered by nuclear fusion in its core, but surrounded by a low-density, cooler atmosphere. - Atmosphere on Titan: a layer of cool, low-density gas confined close to the surface of Titan, one of Saturn's moons.

continuous spectra

Spectrum in which the radiation is distributed over all frequencies, not just a few specific frequency ranges. A prime example is the blackbody radiation emitted by a hot, dense body.

4 to 2, followed by 2 to 1, is possible. Downward transitions can be any number of levels.

Suppose an electron in some atom absorbs energy and transitions from level 1 up to level 4. Which of the following is a true statement regarding the electron transitions back down to level 1?

Why are gamma rays generally harmful to life-forms, but radio waves generally harmless?

The reason why gamma rays are so generally more dangerous and harmful to life than visible light or radio waves is that each gamma-ray photon typically carries millions, if not billions, of times more energy than a photon of visible radiation. Gamma rays operate at a much higher frequency than visible light or radio waves and since gamma rays have shorter wavelengths, they produce much more energy. Radio waves are generally harmless because the amount of energy/information is so minuscule that it barely reacts with other lifeforms on their molecular level.

spectroscopy

The study of the way in which atoms absorb and emit electromagnetic radiation. Spectroscopy allows astronomers to determine the chemical composition of stars.

an absorption spectrum

The visible spectrum of sunlight reflected from Saturn's cold moon Titan would be expected to be

Each type of spectrum is unique in the way the wavelengths (colors) of light are distributed. The continuous spectrum shows a continuum of all the colors, whereas the emission spectra show only specific lines of emitted color. The absorption spectra show only small black ranges where specific colors have been absorbed away.

There are three general types of spectra: continuous, emission, and absorption. Each is characterized by a different distribution of the wavelengths (i.e., colors) of radiation. Sort the images of the three types of spectra into the appropriate bins.``

Kirchhoff's laws

Three rules governing the formation of different types of spectra. 1. A luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum of radiation. 2. A low-density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the chemical composition of the gas. 3. A cool, thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. Once again, these lines are characteristic of the composition of the intervening gas—they occur at precisely the same wavelengths as the emission lines produced by that gas at higher temperatures.

Thermal velocities

most cases the line is Doppler shifted just a little

colli-sional broadening

other broadening mechanisms do not depend on the Doppler effect at all. For example, if electrons are moving between orbitals while their parent atom is colliding with another atom, the energy of the emitted or absorbed photons changes slightly, blurring the spectral lines. This mechanism, which occurs most often in dense gases where collisions are most frequent, is usually referred to as