(1) illustrates the fundamental difference between emission nebula and reflection nebula. Before Hubble, diffuse nebulae composed of gas and dust have been found in the Milky Way, some of which are emission spectra on dark background and some are absorption spectra on seven-color band background. The nebulae with these two different spectra are called emission nebulae and reflection nebulae respectively. 1922, Hubble published the article "A General Study of Diffuse Galactic Nebulae", and correctly pointed out that these two nebulae were produced because there were luminous stars with different spectral types. In the emission nebula, there are stars with high surface temperature with spectral types of O, B0 and B 1, while in the reflection nebula, there are only stars with low surface temperature. From B2 to M ... In the late 1930s, Danish astronomer Strangen put forward the theory that hydrogen in high-temperature star ionization nebula formed emission nebula, which further explained this problem. As for the reflection nebula, because the low-temperature stars in it are not enough to excite the hydrogen ions in the nebula and produce the emission spectrum, the nebula simply reflects the light of the stars, so its spectrum is only a simple copy of the star spectrum (absorption line spectrum on the seven-color background).
(2) It is first discovered that some galactic nebulae are remnants of supernova explosions. 1928, he pointed out in his article Nova or Temporary Star that the Crab Nebula is expanding rapidly. He measured its expansion speed and calculated that it would take 900 years to expand from a very small volume to the present scale. At the same time, considering the corresponding position, he boldly proposed that the diffuse nebula in the Milky Way is the remnant of the recorded supernova explosion in China in the Song Dynasty. This view has now been accepted.
(3) In1930, the surface brightness profile of elliptical galaxies (how the brightness decreases outward from the center) was accurately measured for the first time, and the surface brightness profile model he established was later called Hubble luminosity profile of elliptical galaxies.
(4) Take the lead in carrying out detailed research on different kinds of stars and star groups in nearby extragalactic galaxies. Among them, the discovery and study of Cepheid variables in these galaxies constitute the most important method to measure the distance between these galaxies. 1932, he identified some globular clusters in the Andromeda nebula and found that their average luminosity seems to be four times darker than that of globular clusters in our galaxy. Later, German astronomer Budd discovered that this was actually caused by the improper distance of Andromeda nebula measured at that time.
(5) In 1935 and several papers published later, we studied the asymmetry of dust belts in galaxies, determined which side of galaxies is closer to us, and then determined the rotation direction of the spiral arms of these galaxies.
(6) Together with the German astronomer Bud * * who works at Mount Wilson Observatory, we studied the Yuki galaxy and the celestial galaxy, and found that they are both short elliptical galaxies, that is, elliptical galaxies with relatively low luminosity.