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In a planetarium, you can hurtle through a day in less than two minutes! In the 1920s, inventors at the Carl Zeiss Optical Company invented a rotating projector that casts stars on the inside of a dome-and the modern planetarium was born. Turning the projector one way shows the rising and setting of the sun, moon, planets, and stars in a 24-hour day. Turning it another way shows the changing positions of the stars over the course of a year, and what the sky might look like in the past of future.

The first astronomer to identify a pulsar wondered if it was a signal from little green men. Jocelyn Bell was working on her Ph.D. in astronomy at Cambridge University, where she helped build a huge radio telescope. In 1967 the telescope detected a regular pulse (every 1 1/3 seconds). The regular radio waves made Bell's group wonder if the signal was coming from intelligent life, and they even started calling it LGM, short for "little green men." When a second pulsing signal was discovered months later, they knew there couldn't be two groups of little green men signaling, and the name "LGM" was replaced by "pulsar." Bell concluded that these pulsars were a kind of rotating neutron star. Every time the star spins, it flashes a radio signal.

You can see a crab in the sky that is six miles wide. The beautiful Crab Nebula is the cloud of gas left by a supernova explosion. The Crab Pulsar is a neutron star that pulses in the center of the nebula. This star, which is six miles across, has more mass than the sun. Of more than 500 pulsars discovered, only two (the Crab and the Vela) flash visible light and a radio signal.

The universe might be made up of MACHOs and WIMPs. Some scientists think that cold dark matter makes up most of the universe, either in the form of black or brown dwarfs. But others believe that it's made of WIMPs (weakly interacting massive particles) and MACHOs (massive compact halo objects). Because this matter is undetectable, don't expect a definitive answer any time soon.