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Recap – Our Solar System. Name the most important characteristics of Jupiter, Saturn, Neptune, Pluto. What is the difference between asteroids and comets ? How do you explain the tail of a comet ? What are the meteorites ? How are they different from meteoroids ?. Stars.

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Recap – Our Solar System

  • Name the most important characteristics of Jupiter, Saturn, Neptune, Pluto.

  • What is the difference between asteroids and comets ?

  • How do you explain the tail of a comet ?

  • What are the meteorites ? How are they different from meteoroids ?

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  • Our Sun: structure, atmosphere

  • Other stars: properties, measurements

  • Binaries and Variables

  • Observing stars formation

  • Modelling the formation of our planetary system

  • Other planetary systems

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The Sun

  • Located at about 150 million km (1 Astronomical Unit -AU) from us, the Sun’s radius is about 109 times larger than Earth’s radius

  • The density of the Sun is about a quarter of Earth’s, due to 73.5% hydrogen and 25% helium (plus carbon, oxygen, nitrogen, neon and iron).

  • The surface temperature is about 5500oC. The visible light (and also UV, IR) is emitted by the photosphere (the layers above are transparent to visible light).

  • The layer above photosphere is the chromosphere (or the corona), which emits radio, microwaves, UV, X and gamma radiation, plus a variety of elementary particles.

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Sun’s Source of Energy

  • Models of the solar interior show that the temperature in the center reaches about 15 million degrees.

    • At this temperature the protons fuse together to form deuterium.

    • The weak nuclear force helps in this process (with the release of neutrinos and positrons).

    • Deuterons with protons form He3 nuclei (with a release of photons), which collide to form He4 nuclei (alpha particles), with the release of two protons.

  • To maintain the current luminosity the Sun fuses about 600 million metric tons of protons per second.

  • The Sun started shining almost 5 billion years ago and will continue for another 5-6 billion years.

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Modeling the Sun

  • The Sun’s interior zones along the radius:

    • core(20%)

    • radiative (51%)

    • convective(29%)

      Radiation and particles diffuse outwards very slowly

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The Photosphere

  • Photosphere is 500 km thick

  • Small-scale structure of temporary 1000-2000 km-wide granules of hot gas; super and giant sizes.

  • Sunspots are cooler (4000oC) dark areas associated with magnetic fields which inhibit convection. Suspots appear in pairs or groups.

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The Corona

  • The corona (or chromosphere) is 2,000-10,000km thick. It appears pink-red during total solar eclipses.

  • Corona contains plasma which radiates in radio, UV and X. Its temperature reaches about 1 million degrees because of electric currents and discharges:

    • Spicules: flame-like columns of hot gas 10,000km high, 5-10 minutes long

    • Filaments: denser clouds of gas suspended by magnetic fields ; they are up to 200,000km long prominencessomeshort and some long-lived (weeks, months).

    • Solar flares: about 1 hour long explosions equivalent to billions of nuclear bombs. They radiate primarily UV and X, plus plasma (particles such as electrons and protons). In magnetic fields they form loop-like structures, which “snap” producing Coronal Mass Ejections (CME).

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Solar Wind

  • Our Sun looses about 1 million metric tons per second through the solar wind, which blows past planets at high speeds.

  • Because of the Sun’s rotation this wind takes a spiral form and they reach past Neptune (100 AU, the size of the heliosphere).

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The Solar Cycle

  • Solar activity has a 11-year cyclic variation

  • Sunspots grow in numbers and after 11 years grow again with changed polarity

  • Cycles are controlled by the magnetic field which in turn is due to the convection currents.

  • Maxima in solar activity are known as magnetic storms, which influence power lines, telephone and radio communication and produce stronger aureoras.

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Distances to Stars

  • Brightness=apparent magnitude is measured through telescopes

  • Absolute magnitude is related to the star’s internal processes (can be related to star’s oscillations or can be studied through the star’s spectrum)

  • Knowing apparent and absolute magnitude one can determine the Distance to the star.

  • Only for near stars another method to determine the distance is by trigonometrical (annual) parallax .

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Temperature of Stars

  • Temperature at star’s surface is related to the peak wavelength in the star’s continuous spectrum. If known, together with brightness, it allows the calculation of the star’s radius.

  • Hertzsprung-Russell diagrams display stars luminosity vs. temperature

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Stellar Motion

  • Radial velocity – from Doppler effect measurements

  • Transversal velocity – from angular shift (if distance to the star is known one can obtain velocity)

  • Ex. Barnard’s star has 108/90 km/s, which gives a space velocity of 140km/s

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Binary/Multiple Stars

  • Visual Binaries: two stars rotate around the center of mass of the system. About 66% of the stars in our galaxy are Binaries.

  • Multiple Stars examples:

    • Epsilon Lyrae (double-double),

    • Mizar/Alcor in the Plough (or Big Dipper) group.

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Variable Stars

  • Extrinsic Variables

    • Eclipsing binaries – studied through brightness observations

  • Intrinsic variables – when variability is due some internal process:

    • Pulsating variables: Cepheid variables, which are yellowish giants pulsing in 1-80 days; the greater the luminosity the longer the pulse (because of size).

    • Eruptive/cataclysmic variables:

      • Nova– in binaries with a white dwarf (up to million times more luminous)

      • Supernova– at the end of a star life (equivalent to the luminosity of a medium galaxy). Last in our galaxy in 1604.

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Stars Formation

  • Interstellar space has about one hydrogen atom per cm3. Interstellar clouds can be millions of times denser. Some clouds can be between 100,000-300,000 light-years in diameter (larger than galaxies)

  • The process which leads to the formation of a star is extremely slow (millions of years) at temperatures around –263oC.

  • Stars appear when a cloud of cosmic gas and dust collapses under gravitation to form extremely dense objects which are the stars “embryos”.

  • Young stars are extremely bright and emit primarily UV photons. UV radiation can ionize atoms in the cloud, which later recapture electrons and emit de-excitation radiation. This makes the cloud very luminous in various colors.

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The Great Nebula in Orion

  • One of the nearest “star nurseries” is the Orion Nebula. It is at about 1500 light-years from us and has a diameter of about 20 light-years. In 1936 astronomers observed with a lot of excitement the birth of a new star baptized Fu-Orionis.

  • Hubble Space Telescope showed that Orion contains hundreds of young stars, while IR observations showed that another batch of stars is forming behind the visible nebula

  • The analysis of the spectra emitted from Orion shows the existence of hydrogen and condensed ammonia, water and many other molecules (about 100 types), which play an important role in the gravitational collapse.

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Three “Star Nurseries”

Orion Nebula

Eagle Nebula

Tarantula Nebula

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More about Cold Star Nurseries

  • Young stars can be observed through optical telescopes too. They always appear surrounded by a fuzzy “atmosphere”, when clouds of dust (carbon and silicates grains) backscatter the star’s light. In other cases, dust clouds completely hide stars from us, as they absorb all visible radiation.

  • Our galaxy contains about 5000 cold star nurseries with masses between 100,000-10 million solar masses and diameters between 50-300 light-years.

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Hot Star Nurseries

  • Data about stars formation can be also obtained from X-rays telescopes.

  • The HEAO-1 satellite showed that (only) 6000 l-y away from us there is a bubble of hot gas where stars are being formed at a rate much higher than in Orion.

  • The bubble is about 1,200 light-years wide and it was formed in a succession of 30-100 stellar explosions (supernovas).

  • Our Galaxy is believed to have about 200 hot star nurseries.

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The Birth of Our Planetary System (I)

  • Kant, Laplace emitted in the 18th century the model in which planets were formed in the spinning disc. In the solar system all planets (except Mercury and Pluto) move in a common plane, but the angular momentum is low for the Sun although it has 99.8% of the total mass of the system.

  • Simulations performed in 1970s showed that the spinning disc becomes extremely thin and brakes into planetesimals of 5-10 km in size. Terrestrial planets and the core of giant planets form through collisions, with the larger planetesimals mopping up the smaller ones in about 100 million years. These early collisions influenced the planets rotation speed and the tilt of their axis.

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The Birth of Our Planetary System (II)

  • The stellar wind sweeps away most of the nebula, but at larger distances (larger than 5AU for our solar system) the temperature is low enough for water to freeze therefore increasing the size of the solid protoplanets. At this distance nebula material is captured in the gravitational field of the protoplanets and forms huge gaseous envelopes.

  • The condensation of the jovian planets resembles that of the star with a spinning disc of material. Many of the moons were formed from that disc, while others were plantesimals captured by the planets.

  • The terrestrial planets form their atmosphere through the outgassing of material from the interior. Many craters on these planets were formed by plantesimals impact in the first few hundred million years.

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Other Planetary Systems (I)

  • Planets are expected to be quite common, but observing planets is difficult.

  • If our solar system would be observed from 4 light-years, the angular separation between our Sun and Jupiter would be 4 arcsec, which could be resolved with a normal telescope if both were equally bright. Jupiter is a billion times fainter than the Sun and would be invisible. In IR it is a bit better but poorer resolution.

  • Therefore, until recently, planets in other systems were seen only indirectly through the perturbations in the star’s movement or luminosity.

  • Today it is possible to observe directly some extra-solar planets by blocking the direct view of the corresponding star.

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Other Planetary Systems (II)

  • The first planet outside our solar system was observed in Geneva in 1995. It was about half of Jupiter rotating around the star 51 Pegasi (at about 40 light-years from us) in only 4.2 days. That made the distance between the two 0.05AU , or about one eighth of the Mercury-Sun distance.

  • Since then more than 400 planetary systems were discovered. Most planets, with masses between 0.4-11 Jupiter masses, orbit stars at distances under 1AU. Only a few of them move at distances of several AU, which we would expect from the study of our solar system. It seems that the Universe has a variety of planetary systems.