Kinds of Interstellar Matter

Kinds of Interstellar Matter

The matter in space between stars comes in several different forms. Each type comes in regions of differing sizes and has different densities and temperatures. In the second line HI stands for neutral hydrogen, or hydrogen atoms with one proton and one electron.

Average                 Density

Temperature

Size

Mass

Diffuse gas	0.001 to 0.1 particles per cm3	10 to 106 K	Size of the galaxy
HI clouds	10 to 100 particles per cm3	80 to 120 K	Several parsecs	Several hundred solar masses
Globules	103 to 106 particles per cm3	10 K	Several light-years (maximum of 15 light-years)	10 to 100 solar masses
Molecular clouds	100 particles per cm3	10 K	Several tens of light-years to many tens of parsecs	100 to 106 solar masses

Spectral Types of Stars

Spectral Types of Stars

Astronomers categorize stars according to the the characteristics of the light that the stars emit, which are related to the stars’ temperatures. O stars are the hottest stars, and M stars are the coolest. Our Sun is a G star.

Spectral Class

Effective Temperature Star Color

Principal Characteristics

O	25,000 K Blue star	This stage is characterized by lines of helium, oxygen, and nitrogen in the spectrum of the photosphere. O stars are extremely hot, very bright stars that emit large amounts of ultraviolet radiation.
B	11,000 K - 25,000 K White-blue star	In this group the lines of helium in the spectrum reach a maximum intensity and then fade. The intensity of the hydrogen lines regularly increases in all the subdivisions of stage B. Type B stars are typically represented by the star Epsilon Orionis.
A	7500 K - 11,000 K White star	This group comprises the so-called hydrogen stars. The spectra of these stars is dominated by absorption lines of hydrogen. Sirius, the Dog star, is a typical type A star.
F	6000 K - 7500 K Yellow-white star	This group is composed of stars characterized by an elevated intensity of the H and K lines of calcium and of lines characteristic of hydrogen. A notable star in this category is Delta Aquilae.
G	5000 K - 6000 K Yellow, solar star	This group is composed of stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of numerous metals, in particular iron, are also present. The Sun belongs to this group, and therefore G stars are frequently called solar stars.
K	3500 K - 5000 K Orange-yellow star	This group comprises stars having strong calcium lines and lines indicating the presence of other metals in their spectra.The violet light of class K stars is less intense than the stars' red light. This group is typically represented by Arcturus.
M	3500 K Red star	This group is composed of stars whose spectra are dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectra is less intense than that of K stars. The star Orionis is typical of this group.

Characteristics of the Planets

Characteristics of the Planets

Characteristic

Mercury

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto*

Equatorial radius (Earth radii†)	0.3825	0.9488	1	0.5325	11.21	9.449	4.007	3.883	0.1874
Equatorial inclination (degrees)	0.01	2.64	23.5	25.2	3.13	26.7	82.2	28.3	57.4
Mass (Earth masses‡)	0.0553	0.8150	1	0.1074	317.8	95.16	14.54	17.15	0.0023
Average density (g/cm3)	5.4	5.2	5.5	3.9	1.3	0.69	1.3	1.6	1.8
Rotational period (days)	58.6	-243	1	1.03	0.414	0.444	-0.718	0.671	-6.4
Orbital period (years)	0.2408	0.6152	1	1.881	11.86	29.46	84.01	164.8	247.9
Average distance from the Sun (AUs)	0.3871	0.7233	1	1.524	5.203	9.59	19.10	30	39.30
Orbital eccentricity (ratio)	0.206	0.00674	0.0167	0.0935	0.0489	0.0576	0.0497	0.00995	0.248
Orbital inclination (degrees)	7	3.39	0.0003	1.85	1.30	2.49	0.772	1.77	17.2
Moons (number)	0	0	1	2	63	60	27	13	3
*Reclassified as a dwarf planet by the International Astronomical Union in 2006 †Planet's radius expressed as a multiple of Earth's radius (6,378 km) ‡Planet's mass expressed as a multiple of Earth's mass (5.974×1024 kg)

Telescopes of the World

Telescopes of the World

Large telescopes are found all over the world. Most large telescopes are based on high mountaintops, where Earth's atmosphere is thinner and does not interfere as much with light from distant stars.

Telescope Name

Aperture (m)

Year of Startup

Observatory

Large Binocular Telescope	equivalent to 11.8 m (uses two 8.4-m primary mirrors)	2007	Mount Graham International Observatory (Mount Graham, Arizona, United States)
Gran Telescopio Canarias	10.4	2006	Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain)
Southern African Large Telescope (SALT)	~10.0	2005	South African Astronomical Observatory (Sutherland, South Africa)
Keck I	10.0	1993	W.M. Keck Observatory (Mauna Kea, Hawaii, United States)
Keck II	10.0	1996	W.M. Keck Observatory (Mauna Kea, Hawaii, United States)
Hobby-Eberly Telescope (HET)	9.2	1999	McDonald Observatory (Mt. Fowlkes, Texas, United States)
Subaru Telescope	8.2	1999	National Astronomical Observatory of Japan (Mauna Kea, Hawaii, United States)
Antu1	8.2	1998	Very Large Telescope array (VLT) (Cerro Paranal, Chile)
Kueyen1	8.2	1999	Very Large Telescope array (VLT) (Cerro Paranal, Chile)
Melipal1	8.2	2000	Very Large Telescope array (VLT) (Cerro Paranal, Chile)
Yepun1	8.2	2000	Very Large Telescope array (VLT) (Cerro Paranal, Chile)
Frederick C. Gillett/Gemini North	8.1	2000	Gemini Observatory (Mauna Kea, Hawaii, United States)
Gemini South	8.1	2001	Gemini Observatory (Cerra Pachon, Chile)
Multiple Mirror Telescope (MMT)	6.5	2000	Fred Lawrence Whipple Observatory (Mount Hopkins, Arizona, United States)
Walter Baade/Magellan I	6.5	2000	Las Campanas Observatory (La Serena, Chile)
Landon Clay/Magellan II	6.5	2002	Las Campanas Observatory (La Serena, Chile)
Bolshoi Teleskop Azimutalnyi (Bolshoi Azimuthal Telescope)	6.0	1975	Special Astrophysical Observatory (Nizhnij Arkhyz, Russia)
Large Zenith Telescope (LZT)	6.0	2004	University of British Columbia (UBC) Liquid-Mirror Observatory (British Columbia, Canada)
Hale Telescope	5.1	1948	Palomar Observatory (Palomar Mountain, California, United States)
1Each of these telescopes can be used separately or as part of the Very Large Telescope array.