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Figure 9.23: An artist’s concept of our heliosphere as it travels through our galaxy. The solar wind – a thin stream of electrically charged gas from the sun – creates a bubble as it moves with the sun through interstellar space called the heliosphere. The wind travels at an average speed of 300–700 km/s until it reaches the termination shock. At this point, the speed of the solar wind drops abruptly as it begins to feel the effects of interstellar wind. The heliosheath is the outer region of the heliosphere, just beyond the termination shock. The boundary between solar wind and interstellar wind is the heliopause, where the pressures of the two winds are in balance. As the heliosphere ploughs through interstellar space, a bow shock forms, similar to the wave which forms as a ship ploughs through the ocean. Two Voyager spacecraft have made their way out of the heliosphere.


Instead, the orbital speed does not decline with distance (red solid curve). Te mass M observations for galaxies based on the light that they emit are far too low to explain the velocity observations. To explain the missing mass, the existence of dark matter is postulated. Masses of galaxies can be determined from the observed light.


First, the total mass in stars is determined. One adds up the total amount of observed starlight, and then uses the galaxy’s distance (or redshift) to convert the observed light into the true amount of light emitted by the galaxy. Te ratio between mass and light now depends on the colour (or age) of the stellar population, which is used to determine the total mass in the stars. Second, galaxies also contain gases, which contribute to the total mass of a galaxy. In nearby galaxies, for instance, the amount of gas can be directly measured using radio telescopes. In the 1970s, Vera Rubin and others pioneered detailed


measurements of the orbital speeds of nearby galaxies. Teir measurements showed that the rotation velocity with distance


Figure 9.24: The closest white dwarf star to Earth, Sirius B (the small blue star on the right), discovered in 1862, is buried in the glow of the brightest star in the night sky, Sirius, located 8.6 light-years away in the winter constellation Canis Major, which represents the bigger dog following Orion, the hunter in Greek mythology. The name Sirius is coined from the Greek term Σείριος (Seirios), which means ‘glowing’. Sirius is also known as the Dog Star. It is closest to the Earth in July and August, which has led to the phrase “dog days of summer”. If you wish to spot it, observe the southern sky on a late winter evening, and you will see it slightly towards the south-east.


Sirius B, also known as The Pup, is smaller than the Earth, but much denser with a gravitational field 350,000 times greater than Earth’s, meaning that 3 gm of matter (roughly a sugar cube) would weigh 1,000 kg. The original primary star of Sirius B, about five sun masses, consumed its hydrogen and helium resources around 120 million years ago and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf.


Sirius is one of the 27 stars on the Brazil flag, and depicts the state of Mato Grosso. It is believed that Sirius Black from the Harry Potter books, who has the unique ability to transform into a black dog, could have been inspired by Sirius B.


308


increased, and then unexpectedly remained constant, even beyond the visual extent of the galaxy. If gravity in the outskirts of galaxies behaves like we observe it here on Earth, then for V(R) to be nearly constant, M(R)/R must be nearly constant, implying that there must be more mass in the outskirts of galaxies than just the stars and gas. Tis difference between the observed and predicted rotation curve of spiral galaxies as sketched in Figure 9.22 provided the first evidence for dark matter.


9.1.7 A Star’s Fate


A star’s fate depends on how massive it was at its birth. When a star of the moderate size of our sun finishes its


nuclear fusion burning hydrogen to helium, it swells and becomes a red giant. Eventually, the core collapses, followed by a thermo-nuclear reaction which blows off the outer layers, forming a planetary nebula. Te core becomes a white dwarf. Te time from birth to death is roughly 10 billion years.


NASA/Goddard/Walt Feimer


NASA, ESA and G. Bacon (STScI)


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