*星の明るさが違うのは何故？
夜空を見上げると明るい星から暗い星まで様々な星が見えます。星の明るさは「星が発する光の量」と「地球からの距離」で変わります。夜空で輝く星は月や惑星を除くと太陽のように自ら光を発している星（恒星）ばかりです。太陽は地球からの距離が近いので、太陽が出ている時は他の星々は見えませんが、宇宙には太陽よりも強い光を放つ星は沢山あります。全天で最も明るい星であるおおいぬ座のシリウスは直径が太陽の2.1倍、オリオン座のベテルギウスは太陽の400倍以上もあると言われています。星の明るさは等級で表し、数字が大きくなるほど暗くなります。最も明るいシリウスは-1.4等級、太陽は-26等級に相当します。肉眼で見える最も暗い星は6等級です。惑星は太陽の光を反射して輝きますが、地球からの距離が近いので星座の星々よりも更に明るく見えるのです。
*Understanding the Brightness of Stars in the Night Sky
When we gaze up at the night sky, we observe stars of varying brightness, from the brilliantly shining ones to the fainter ones. The brightness of a star is determined by the amount of light it emits and its distance from Earth. In the night sky, the stars that twinkle are primarily stars that emit their own light, much like the Sun, except for the Moon and planets.
The Sun, being relatively close to Earth, outshines other stars in our daytime sky, rendering them invisible. However, the universe is replete with stars that emit light far brighter than the Sun. Sirius, the brightest star in the entire sky, has a diameter 2.1 times that of the Sun, while Betelgeuse in the Orion constellation is said to be over 400 times the Sun's size.
The brightness of stars is expressed in magnitudes, with larger numbers indicating fainter stars. Sirius, the brightest, has an apparent magnitude of -1.4, while the Sun is equivalent to -26. The dimmest stars visible to the naked eye have a magnitude of 6. Planets, on the other hand, appear brighter than the stars in constellations because they reflect the Sun's light and are much closer to Earth in terms of distance.
