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Science and the Universe

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Nature of Astronomy

The study of the objects that lie beyond the atmosphere of our planet EarthThe study of the processes by which these objects interact with one anotherHumanity’s attempt to organize the knowledge about our universe into a clear history of the universe from the instant of its birth to the present time and beyondAnd much more…

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Nature of Science

Science is a method (or process) by which one attempts to understand the material universe (nature) and how it behavesThe ultimate judge in science is the experiment or observation: what nature itself reveals

Science accepts nothing on faith

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Scientific Method Involves …

Many (repeated) observations over a period of timeAnalyzing the trends and patternsThe formulation of models of the particular phenomenaTesting the models with further experiments and observationsIteration of the above process

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Concrete Example of Scientific Process

Ancient astronomers built a geocentric model of the universe Better observations required the model to add circles after circles to the movements of the planets to keep the Earth at the centerThe geocentric model eventually could no longer explain all the observed facts and was abandoned in favor of a heliocentric model, which fit the experimental evidence better

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Some Remarks about Scientific Process

New models/ideas are called hypothesesContrary to common beliefs, much is yet to be learned in astronomy (and physics in general)New phenomena are observed constantly, and new hypotheses needed to explain theseSome observational facts are very well understood, but others remain “mysterious”Typically, new ideas are difficult to test either because the amount of data is small, or because they require observations that are difficult based on current technology

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Some Remarks about Astronomy

Astronomy is an observational scienceIt’s impossible for an astronomer to experiment with planets or stars in a laboratory on Earth

Much of astronomy is also a historical scienceThe events being observed have already happenedThis is similar to a detective trying to solve a crime that occurred before the detective ever got to the scene

Astronomers must prove their case “beyond a reasonable doubt”

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Laws of Nature

Over the centuries, from experiments and observations, scientists have extracted certain fundamental principles, called scientific lawsLaws are the same everywhere in the universeAstronomy would be difficult or even impossible without such lawsLaws (models and theories) are constantly refined by scientists to follow new, and typically more comprehensive, observations of natureDescribing the laws of nature requires a precise language: mathematical models and equations

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Numbers in Astronomy

Distances in astronomy are hugeIt is convenient to use the powers-of-ten notation, sometimes called scientific notationExamples

5,200,000 = 5.2 millions = 5.2 x 106

0.000125 = 1.25 x 10-4

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Light Years

Astronomers use a convenient unit called “light-year” to describe distances in the universeOne light-year, abbreviated as 1 LY, equals the distance that light travels in one yearThis unit is useful because light (of any color)

always travels at the same speed in vacuum (empty space)has the largest speed at which anything can travel

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Speed of Light (in Vacuum)

Light travels at the amazing speed of 300,000 kilometers per second (1 kilometer = 0.6 mile) or

300,000 km/s = 3 x 105 km/s 3 x 108 m/s 1,080,000 km/hour

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One Light-Year (1 LY)

60 seconds in 1 minute3 x 105 km/s x 60 s/min = 1.80 x 107 km/min

60 minutes in 1 hour1.80 x 107 km/min x 60 min/hour = 1.08 x 109 km/hour

24 hours in a day1.08 x 109 km/hour x 24 hour/day = 2.592 x 1010 km/day

365.24 days in a year2.592 x 1014 km/day x 365.24 day/year = 9.46 x 1012 km/year

1 LY = 9.46 x1 LY = 9.46 x 10 101212 km km (= 5.9 x 10(= 5.9 x 101212 mi) mi)

Powers of Ten movie

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How We Learn about Distant Objects

Information about distant objects comes to us almost exclusively via various forms of lightAll such light travels at the speed of light (namely, 1 LY every year)This leads to two consequences …

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Consequence 1

The fact that light takes time to travel sets a limit on how quickly we can learn about events in the universe

If a star is 10 LY away, the light we see from it tonight left that star 10 years agoIf that star exploded today, we would learn about it only 10 years from now

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Consequence 2

Since light takes time to travel, an observation of distant objects in the sky is actually an observation of what happened in the past

The further away the objects we look at, the more ancient they areThis is a potential benefit in disguise, providing a way to study the history of the Universe

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An Imaginary Journey from Home

EarthPlanetsSolar SystemStar ClustersGalaxyGalaxy ClustersGalaxy Super-clustersThe Universe!

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Planet Earth

A nearly spherical planet about 13,000 km in diameterDistinguished by the large amount of liquid water it has, which covers 2/3 of its surfaceSupports “intelligent life”Has one natural satellite: the Moon

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The Moon

The Moon’s diameter is about 3476 kmThe Moon-to-Earth distance is about 30 times the Earth’s diameterLight travels from the Moon to the Earth in about 1.3 seconds Photo by John French, Abrams Planetarium

Michigan State University

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More about the Earth

The Earth revolves around the Sun The period of revolution is 3 x 107 s (1 year)

The average Earth-to-Sun distance is about 150 million km

This defines a unit of distance called astronomical unit (AU)Thus 1 AU = 150 million km

The Earth travels at about 110,000 km/hour in its orbit around the SunLight takes 8 minutes to travel from the Sun to the Earth

Our latest news from the Sun is always 8 minutes old!

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Solar System (1)The solar system consists of the Sun at its center and nine planets orbiting the Sun, plus other smaller objects

The Earth is one of the nine planets

A planet is a body of significant size that orbits a star and does not produce its own light

The Sun is a starIts diameter is 1.5 million km

A star is large body which (at some point during its life) produces light by nuclear reactions

Solar System (2)The other 8 planets are visible from Earth because of the (Sun’s) light they reflect towards us (albedo)The 9 planets are located at enormous distances from the Sun as illustrated here

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Being the largest planet in the solar system, Jupiter has a diameter of 143,000 km

11 times Earth’s sizeIts distance to the Sun is about 5 AULike many other planets, it has a number of natural satellites or moons

Jupiter

              

                

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The Milky Way (1)The Sun is our local starOther stars are also suns: typically enormous balls of glowing gas that generate vast amounts of energy and light via nuclear reactionsOther stars look small and faint compared to our sun because they are extremely distant from usThe nearest star beyond the Sun is Proxima Centauri, which is 4.3 LY awayThe stars we see at night are part of a gigantic complex of stars called the Milky WayThe Milky Way is a Galaxy

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The Milky Way (2)

The Sun is about 30,000 LY from the center of our GalaxyFrom our location within the Galaxy, we cannot see through its far rim because the space between stars is not empty, but contains (an extremely sparse distribution of) interstellar dust or gas which absorbs visible lightThe interstellar gas and dust are believed to be the raw material for future formation of stars

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The Milky Way (3)There is indirect evidence that much of our Galaxy is made of material that cannot be observed with present-day instruments

Thus, much of our Galaxy seems to consist of dark matter

Stars in a number of places form clusters, each containing up to hundreds of thousands of stars and taking up a volume of space hundreds of light years acrossStars have a long, but not infinite, life (believed to be billions of years)

They die or collapse in spectacular ways

Some Nearby Objects

Our Galaxy, the Milky Way, is not aloneMany other galaxies have been discovered and studied, and most are at enormous distances from ours

So many galaxies have been found that it would be foolish to give them all proper names -- they are usually given a catalog number

The Magellanic Clouds are two of the small galaxies nearest to oursThe nearest large galaxy is Andromeda, or M31

The two Magellanic Clouds

NOAO/AURA/NSF

Andromeda NOAO/AURA/NSF

Local Group of GalaxiesM31, along with our Galaxy, is part of a small cluster of over 40 galaxies which is called the Local GroupAt distances of about 10 to 15 million LY, one finds other small galaxy groups or clustersAt about 50 million LY, one finds a very large cluster containing thousands of galaxies, called the Virgo ClusterSome of the observed clusters appear to form superclustersOur Local Group and the Virgo Cluster are part of one such supercluster which stretches over a diameter of 60 million LY

Center of Virgo Cluster

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And beyond …

At even greater distances, galaxies are too dim to see, and one finds spectacular objects called quasars

Quasars are brilliant centers of galaxies, glowing with enormous intensities

One theory suggests that a giant black hole is swallowing whole neighborhoods of raw material to produce the observed huge and intense emissions

Quasars allow us to probe the universe 10 billion or more LY away and, thus, 10 billion years in the past!

This may bring us closer to the earliest times of the universe, and even its beginning!