"The Milky Way Galaxy contains about 300 (200-400) billion stars. All of these stars, and the gas and dust between them, are rotating about a galactic center. Stars that are farther away from the center move at faster speeds, but take longer to go around it," Kali said.

Francis raised her hand, "The Milky Way Galaxy contains about 200-400 billion stars. All of these stars, and the gas and dust between them, are rotating about a galactic center. Stars that are farther away from the center move at slower speeds and take longer to go around it."

4 points to francis!

Then Sun is located in the outer part of the galaxy, at a distance of 2.7 × 1017 km (1.7 × 1017 mi) from the center. The Sun, which is moving around the center at a velocity of 220 km/s (140 mi/s), takes 250 million years to complete one trip around the center of the galaxy. The Sun has circled the galaxy more than 18 times during its 4.6-billion-year lifetime.

How is the brightness of a star measured?

Mithos raised his hand "Brightness is usually measured by apparent magnitude."

Francis raised her hand, "In astronomy, brightness is measured by the apparent magnitude scale. This is a system that uses the stars themselves as standards rather than using energy units.The magnitude scale was developed by the Ancient Greek astronomer, Hipparchus, around 120 BC. It was originally a rough visual system. The brightest stars were said to be of the first magnitude, the next brightest were second magnitude stars. This continued down to sixth magnitude stars at the limit of naked eye visibility. Note that the brighter stars are associated with the smaller magnitudes. There are five magnitudes between 1 and 6. Since 100 is 102, a little mathematics shows that each whole number value on the magnitude scale differs from the next by a factor of 102/5 (the fifth root of 10 squared). 102/5 is roughly equal to 2.512. In other words, a first magnitude star is 2.512 times brighter than a second magnitude star. A second magnitude star is 2.512 times brighter than a third magnitude star, and so on. The difference between two magnitudes is 2.512 x 2.512 (approximately 6.310). So a first magnitude star is 6.310 times brighter than a third magnitude star."

Francis continued, "Modern stellar magnitudes are often given to two decimal places. For example the magnitude of the star, Deneb is given as 1.25. Aldebaran has a magnitude of 0.85. When the magnitudes of stars were measured accurately using this new definition, some stars were found to be brighter than first magnitude. Arcturus, for example is found to have a magnitude of 0.00. Sirius, the brightest star, has its magnitude given as -1.46."

From the back of the classroom Xavier raises his hand.

There are two ways to measuring the brightness of a star, apparent magnitude is the brightness seen from Earth, and absolute magnitude which is the brightness of a star seen from a standard distance of 10 parsecs (32.6 light years). Stars can be plotted on a graph using the Hertzsprung Russell Diagram.

3 points for all!

The apparent magnitude of the Sun is −26.72. The apparent magnitude of the brightest star in Earth’s night sky, Sirius, is −1.46. The dimmest stars that can be seen from Earth with unaided eyes have apparent magnitudes of about 6.

Astromomers also measure a star’s brightness in terms of its luminosity. A star’s absolute luminosity or intrinsic brighness is the total amount of energy radiated by the star per second. Luminosity is often expressed in units of watts. The Sun’s absolute luminosity is 3.86 × 1026 watts. The absolute luminosity of stars ranges from one thousandth of the luminosity of the Sun to ten million times that of the Sun.

The Sun is a second-generation star, meaning that some of its material came from former stars. Some stars in our galaxy are nearly as old as the expanding universe, which scientists believe originated in the big bang explosion about 15 billion years ago. In contrast, the Sun is only 4.6 billion years old.

The first stars were composed only of the hydrogen and helium produced in the early universe. These stars are called first-generation stars. Although hydrogen is also the main ingredient of the Sun, it contains heavier elements, such as carbon, nitrogen, and oxygen, as well. These elements formed inside first-generation stars that lived and died before the Sun was born. When these massive, short-lived stars used up their internal fuel, they exploded and ejected the heavier elements into interstellar space. The Sun formed from this material, making it a second-generation star.

What kind of reaction takes place in the sun to make it "shine"?