What the difference in firebase-core and firebase-analytics libraray?

Based on google docs you don't need firebase-core anymore:

You no longer need to add the Android library com.google.firebase:firebase-core. This SDK included the Firebase SDK for Google Analytics. Now, to use Analytics (or any of the Firebase products that require or recommend the use of Analytics), you need to explicitly add the Analytics dependency: com.google.firebase:firebase-analytics:17.2.0.

Currently there is no difference.

The firebase-core:9.0.0 has no classes and resources in the aar file and contains the firebase-analytics as you can check in the pom file.

  <modelVersion>4.0.0</modelVersion>
  <groupId>com.google.firebase</groupId>
  <artifactId>firebase-core</artifactId>
  <version>9.0.0</version>
  <packaging>aar</packaging>
  <dependencies>
    <dependency>
      <groupId>com.google.firebase</groupId>
      <artifactId>firebase-analytics</artifactId>
      <version>9.0.0</version>
      <scope>compile</scope>
      <type>aar</type>
    </dependency>
  </dependencies>
</project>

Also in the doc you can find:

com.google.firebase:firebase-core:9.0.0 ->> Analytics

Apparently since June 12, 2018 declaring com.google.firebase:firebase-core is required to use any of the Firebase features (firebase.google.com/support/release-notes/android#update_may_23_2018). According to firebase.google.com/docs/android/setup#available-libraries , firebase-analytics is not even a public module, and is probably not intended to be used directly.