**3. Observing the galaxies and the universe**

In a similar way to stars, galaxies are also categorized along their paths of evolution. Hubble classification of galaxies evolution, the *Hubble Sequence* (**Figure 7**), reviewed here [11] (initial article [12]), provides an observation-based classification of galaxies.

The *Elliptical* galaxies start at spherical (E0; e = 0) to the most common type of elliptical galaxies (E7; e = 0.7). As the galaxy tends to age, central spin tends to send matter away in form of spiral arms.

The second level of classification in the *Hubble Sequence* (and further modifications) is dedicated to the extension and the shape of the *Spiral* arms of the galaxies. As seen in **Figure 7**, two branches of evolution differ in shapes of both the central bulge (whether it keeps spheroid or is barred) and the type of arms evolution. The first type, with spheroid central bulge is classified as Sa, Sb, and Sc along the evolution path. Similarly, *Barred* spiral galaxies are SBa, SBb, and SBc.

**Sa (SBa)** central bulge is bright and prominent.

Arms are tightly wound and smooth.

**Sb (SBb)** central bulge is less bright.

Arms are less tight than above.

**Sc (SBc)** central bulge is smaller and fainter.

Arms are loosely wound (stellar clusters and nebulae).

**Sd (SBd)** central bulge is dim.

Arms are bright and very loose, possible fragmentary arms.

The central bulge of our galaxy, the *Milky Way*, is dominated by a super massive black hole (SMBH), Sagittarius A\* [13]. Sgr A\*'s event horizon image is seen in **Figure 8**. It has an estimated mass of 4.152 106 *M*<sup>⊙</sup> and is the prime of several stars, their orbits helping define its mass. Its observed diameter is 51.8 106 Km, slightly more than the Sun-Mercury maximum distance (<sup>⊙</sup>☿ = 46 106 Km at <sup>⊙</sup> perihelion), which is about 1/3 AU (the mean distance ⊙ ⊕).

Looking outside of the solar system has been largely enhanced with space telescopes. Furthering the capacity of the Hubble space telescope, in 2022, the James Webb Space Telescope (JWST) was activated at Lagrange 2, including the nearinfrared spectrograph (NIRSpec) [14]. Its first images have been no less than revolutionary, giving direct observations of exoplanets and their atmosphere, but also looking further into the past of the universe.

The decades ahead of us promise the enhancement of our understanding of Sun, planets, the stars, black holes, and all other astronomical objects in our universe available to be observed. The observable universe itself just got smaller with JWST activated, and our understanding of the universe and its temporal unraveling is also furthering with every new data gathered. Time itself may also be better understood eventually, who knows?

**Figure 8.** *The event horizon of the SMBH Sgr a\* at the center of the milky way [13].*

*Introduction Chapter: Astronomy DOI: http://dx.doi.org/10.5772/intechopen.108133*
