Oct. 28th - Spacecraft Exploration, Radioactive Dating Flashcards
How can we know about the universe in the past?
Because looking farther into space means looking further back in time, we can actually see parts of the universe as they were long ago, simply by looking far enough away. In other words, telescopes are somewhat like time machines, enabling us to observe the history of the universe
How did we come to be?
Birth of the Universe:
- The expansion of the universe began with the hot and dense Big Bang.
- The universe continues to expand, but on smaller scales gravity has pulled matter together to make galaxies
How did we come to be?
The big bang:
Telescopic observations of distant galaxies show that the entire universe is expanding, meaning that average distances between galaxies are increasing with time
This fact implies that galaxies must have been closer together in the past, and if we go back far enough, we must reach the point at which the expansion began. We call this beginning the Big Bang (around 14 billion years ago)
How did we come to be?
Expansions
- Structures such as galaxies and galaxy clusters occupy regions where gravity has won out against the overall expansion.
- That is, while the universe as a whole continues to expand, individual galaxies and galaxy clusters (and objects within them such as stars and planets) do not expand
Galaxies as Cosmic Recycling Plants:
- The early universe contained only two chemical elements: hydrogen and helium.
- All other elements were made by stars and recycled from one stellar generation to the next within galaxies like our Milky Way.
Life Cycles of Stars:
Many generations of stars have lived and died in the Milky Way.
- Stars born in clouds of gas and dust; planets may form in surrounding disks
- Stars shine with energy released by nuclear fusion, which manufactures elements heavier than hydrogen and helium
- Massive stars explode when they die, scattering the elements they’ve produced in to space
Earth and Life:
By the time our solar system was born, 412 billion years ago, about 2% of the original hydrogen and helium had been converted into heavier elements. We are therefore “star stuff” because we and our planet are made from elements manufactured in stars that lived and died long ago.
How is a star born?
A star is born when gravity compresses the material in a cloud to the point at which the center becomes dense enough and hot enough to generate energy by nuclear fusion, the process in which lightweight atomic nuclei smash together and stick (or fuse) to make heavier nuclei.
Life/Death of a star
- The star “lives” as long as it can shine with energy from fusion, and “dies” when it exhausts its usable fuel.
- A star blows much of its contents back out into space; most massive stars die in titanic explosions called supernovae
Cosmic Recycling
- The returned matter mixes with other matter floating between the stars in the galaxy, eventually becoming part of new clouds of gas and dust from which new generations of stars can be born.
- Galaxies therefore function as cosmic recycling plants, recycling material expelled from dying stars into new generations of stars and planets.
Star stuff:
The early universe contained only the simplest chemical elements: hydrogen and helium (and a trace of lithium). We and Earth are made primarily of other elements, such as carbon, nitrogen, oxygen, and iron. Where did these other elements come from?
Evidence shows that they were manufactured by stars, some through the nuclear fusion that makes stars shine, and most others through nuclear reactions accompanying the explosions that end stellar lives (recycling)
How do our lifetimes compare to the age of the universe?
- Calendars with each month representing the 14 billion year existence of the universe
- Solar system and planet didn’t form until 4.5 billion years ago
- Death of dinosaurs = 66 million years ago
Perhaps the most astonishing fact about the cosmic calendar is that the entire history of human civilization falls into just the last half-minute.
To date, we have sent robotic spacecraft to…
… all the terrestrial and jovian planets, as well as to many moons, asteroids, and comets.
What did Halley figure out from planetary transits?
In 1716, Halley hit upon the idea that would ultimately solve the problem: He realized that during a planetary transit, when a planet appears to pass across the face of the Sun, observers in different locations on Earth would see the planet trace slightly different paths across the Sun.
Comparison of these paths could allow calculation of the planet’s distance—which would in turn allow determination of the AU—through the simple geometry
Only Mercury and Venus can produce transits visible from Earth.
How do robotic spacecraft work?
- The spacecraft we send to explore the planets are robots designed for scientific study.
- All spacecraft have computers used to control their major components, power sources such as solar cells, propulsion systems, and scientific instruments used to study their targets.
- Robotic spacecraft operate primarily with preprogrammed instructions, but also carry radios that allow them to communicate with controllers on Earth.
- Most robotic spacecraft make one-way trips, never physically returning to Earth but sending their data back from space in the same way we send radio and television signals.
