Objects in astronomy can be categorized as being in either the Solar System, the <1>, or the Universe. These three categories serve as useful bins for classifying all astronomy knowledge.
The Earth is a very <2> part of the cosmos. It is a <3> planet that revolves around the sun with seven (or eight) other planets in our solar system. Its average distance from the sun, the <4>, is the basis for measuring distances within the solar system. The radius of our solar system is about <5>, which is the size of Pluto's orbit around the sun.
The sun contains over 99% of the solar system’s <6>. The sun is just an average star. Its only atypical characteristic is that it exists alone -- only <7> of stars are found alone. Most stars are in binary systems or groupings of even more stars that are gravitationally bound.
Our sun is one of <8> stars in the Milky Way galaxy. Our solar system travels in a roughly circular orbit with a radius of <9> around the center of the Milky Way. It makes a revolution in about 250 million years.
The Milky Way is one of about <10> or so galaxies that are part of a gravitationally bound system called the Local Group. These galaxies travel through space orbiting a common center of mass. The Local Group is part of a larger structure known as the Local Supercluster. The universe consists of large "strings" of superclusters called filaments and very sparse regions known as voids. From images like the Hubble Deep Field, astronomers estimate that there are about <11> galaxies in the universe.
@ qu.1.1.blank.1=Milky Way Galaxy, Local Group, Local Supercluster, Virgo Supercluster@ qu.1.1.blank.2=small, great, important, unusual@ qu.1.1.blank.3=terrestrial, jovian, massive, gaseous@ qu.1.1.blank.4=astronomical unit, parsec, lightyear, 1000 km@ qu.1.1.blank.5=40 AU, 80 AU, 10 AU, 5 AU, 20 AU@ qu.1.1.blank.6=mass, debris, moons, light@ qu.1.1.blank.7=35 percent,20 percent,50 percent,62 percent@ qu.1.1.blank.8=100 billion, 100 million, 100 trillion, 100 thousand@ qu.1.1.blank.9=8 kpc, 80 kpc, 800 kpc, 0.8 kpc@ qu.1.1.blank.10=30, 300, 3000, 3@ qu.1.1.blank.11=100 billion, 100 million, 100 trillion, 100 thousand@ qu.1.1.grader.1=menu@ qu.1.1.grader.2=menu@ qu.1.1.grader.3=menu@ qu.1.1.grader.4=menu@ qu.1.1.grader.5=menu@ qu.1.1.grader.6=menu@ qu.1.1.grader.7=menu@ qu.1.1.grader.8=menu@ qu.1.1.grader.9=menu@ qu.1.1.grader.10=menu@ qu.1.1.grader.11=menu@ qu.1.2.mode=Blanks@ qu.1.2.editing=useHTML@ qu.1.2.question=Objects in astronomy can be categorized as being in either the Solar System, the <1>, or the Universe. These three categories serve as useful bins for classifying all astronomy knowledge.
The Earth is a very <2> part of the cosmos.
It is a terrestrial planet implying that it is small and rocky, while the large gaseous planets are known as <3>. Its average distance from the sun, the <4>,
is the basis for measuring distances within the solar system. The radius of our solar system is about <5>, which is the size of Pluto's orbit
around the sun.
The sun contains over 99% of the solar system’s <6>. The sun is just an average star. Its only atypical characteristic is that it exists alone -- only <7> of stars are found alone. Most stars are in binary systems or groupings of even more stars that are gravitationally bound.
Our sun is one of <8> stars in the Milky Way galaxy.
These stars are arranged in a disk which has a diameter of about <9>.
The Milky Way is one of about <10> or so galaxies that are part of a gravitationally bound system called the Local Group. These galaxies travel through space orbiting a common center of mass. The Local Group is part of a larger structure known as the Local Supercluster. The universe consists of large "strings" of superclusters called filaments and very sparse regions known as voids. From images like the Hubble Deep Field, astronomers estimate that there are about <11> galaxies in the universe.
@ qu.1.2.blank.1=Milky Way Galaxy, Local Group, Local Supercluster, Virgo Supercluster@ qu.1.2.blank.2=small, great, important, unusual@ qu.1.2.blank.3=jovian, super-massive, Saturnian, Plutonian@ qu.1.2.blank.4=astronomical unit, parsec, lightyear, 1000 km@ qu.1.2.blank.5=40 AU, 80 AU, 10 AU, 5 AU, 20 AU@ qu.1.2.blank.6=mass, debris, moons, light@ qu.1.2.blank.7=35 percent,20 percent,50 percent,62 percent@ qu.1.2.blank.8=100 billion, 100 million, 100 trillion, 100 thousand@ qu.1.2.blank.9=30 kpc, 3 kpc, 300 kpc, 0.3 kpc@ qu.1.2.blank.10=30, 300, 3000, 3@ qu.1.2.blank.11=100 billion, 100 million, 100 trillion, 100 thousand@ qu.1.2.grader.1=menu@ qu.1.2.grader.2=menu@ qu.1.2.grader.3=menu@ qu.1.2.grader.4=menu@ qu.1.2.grader.5=menu@ qu.1.2.grader.6=menu@ qu.1.2.grader.7=menu@ qu.1.2.grader.8=menu@ qu.1.2.grader.9=menu@ qu.1.2.grader.10=menu@ qu.1.2.grader.11=menu@ qu.2.topic=Special Units and the Metric System@ qu.2.1.mode=Blanks@ qu.2.1.editing=useHTML@ qu.2.1.question=The large distances mentioned above require astronomers to devise some special tools to express them. These include Special Units, Metric Prefixes, and Scientific Notation. The special units include the astronomical unit, the light year, and the parsec. Metric prefixes succinctly express a power of 10 which is applied to a base unit. Scientific notation is a method of mathematical shorthand which conveniently describes very large and small numbers using powers of 10.| Quantity | Value | in kilo- | in centi- | in mega- |
|---|---|---|---|---|
| Lunar period | 2360621 s | <1> | <2> | <3> |
| One AU | 1.5*1011 m | <4> | <5> | <6> |
| Quantity | Value | in mega- | in kilo- | in centi- |
|---|---|---|---|---|
| 1 Parsec | 206265 AU | <1> | <2> | <3> |
| One Ly | 9.46*1015 m | <4> | <5> | <6> |